1 Head of Department.
2 In cooperation with U. S.
a Cooperative, other divisions, U. of F.
4 In Military Service.
6 On leave.

Potato Diseases in Florida

By A. H. EDDINS, GEO. D. RUEHLE and G. R. TOWNSEND'

Potatoes are grown in Florida during the fall, winter and
spring for local consumption and for shipment to Northern
markets. The commercial potato crop is grown mostly in 6
concentrated areas in the State (Fig. 2). As reported by the

Fig. 2.-Shaded areas indicate principal areas of Flor-
ida in which potatoes are grown for shipment to Northern
markets.

Florida State Marketing Bureau for the 10 seasons 1935-36 to
,1944-45, the annual acreage planted to potatoes averaged 27,700
acres and the average annual shipments were 6,384 carloads.
Many changes have been made in the commercial production
of potatoes during the last few years. Acreages planted to po-
tatoes in the Dade County, Fort Myers and Belle Glade districts
have increased, but about 1/4 of the acreage formerly used for
production of the crop at Hastings is now planted to cabbage.
New high-yielding varieties have supplanted Spaulding Rose
in the Hastings area and are displacing Bliss Triumph in other

sections. Tractor-powered machinery is used in producing the
crop and practically all potatoes are washed, dried, graded and
packed at modern packinghouses before they are shipped to mar-
ket. However, difficulties encountered in growing potatoes-
such as freezes, droughts, excessive rainfall and damage from
diseases and insects-have increased for the State as a whole
by planting potatoes in areas where they have never been grown
before and by increasing production in other areas. That has
been particularly true of certain diseases which have caused
severe losses in some sections where production of the crop has
been expanded recently.
Different diseases of the potato are caused by viruses and by
small parasitic organisms, known as fungi and bacteria, which
grow on or in the tubers and plants. Some diseases are trans-
mitted in infected seed potatoes and reappear in the new crop,
while other diseases are caused by organisms which live in the
soil from year to year and by those which spread from other
host plants to the potato.
The potato is also subject to another group of maladies and
injuries which are due to unknown causes and to unfavorable
environmental conditions or nutritional factors. Some potato
injuries can be avoided by using proper preventive measures.
However, the weather cannot be regulated and potato crops are
sometimes almost completely ruined by severe freezes or heavy
rains which occur at critical stages in the growth of the plants.
The occurrence and destructiveness of diseases of the potato
depend upon the weather and the nature and condition of the
soil in which the crop is grown. Warm, rainy periods favor-
able for the development of some of the worst diseases occur
every year in some sections. .Potatoes are planted in marl, peat,
muck, clay and sandy types of soil which must be farmed dif-
ferently to provide drainage as a protection against certain
diseases during wet seasons. Some soils are infested with bac-
teria which cause brown rot of potatoes and other soils may be
ruined for potato culture after they have become infested with
the fungus causing common scab by planting 1 or more crops
with non-treated seed.
The purpose of this bulletin is to describe the characteristics
of different diseases and injuries which may be present in seed
potatoes or are likely to occur in Florida potato fields, and to
recommend control measures which have proved most effective
in eliminating or reducing the losses from each one.

Potato Diseases in Florida

Diseases Due to Fungi
More than a dozen different fungous diseases attack potatoes'
throughout the world and 8 of these, with the exception of
Xylaria tuber rot, which thus far has appeared only in IDade
County, may be present on potatoes in all areas of Florida every
year. The 2 most important fungous diseases are late blight
and common scab.
Late Blight
Late blight, caused by the fungus Phytophthora infestans
(Mont.) D By., is the most common and destructive disease of
potatoes in humid areas throughout the world. It has been
present in potato fields in Dade County every year since 1935
but the damage usually has amounted to less than 5 percent,
except in 1944 when it destroyed tops of plants early and re-
duced yields about 50 percent (62).2 It has caused losses in
the spring crop at Belle Glade for several years and in 1943
it reduced the yield of the fall crop by about 30 percent. In
the Hastings section, annual losses attributed to the disease
from 1932 to 1945 ranged from 0 to 15 percent, averaging 4.8
percent. Late blight also attacks tomatoes and has caused
serious damage to that crop during the last few years, particu-
larly in the southern part of the State. It has also caused
severe injury to eggplant near Fort Myers (35).
The fungus causing late blight does not live in the soil from
year to year but survives from one season to the next in infected
tubers. Thus, blight is carried from one part of the country
to another in diseased potatoes used for seed. Most seed pieces
affected with blight, when planted, decay before sprouting, while
other pieces sprout and produce weak plants. The fungus grows
into the sprout when an infected seed piece germinates, and when
it reaches the parts above ground it produces spores on the stems
and leaves if enough moisture is present and the temperature
is favorable; this is known as a primary infection (50).
Spores are formed on branched fruiting stalks or conidio-
phores which grow out of stomata on infected leaves and also
through lenticels on infected stems and tubers of potato plants.
When placed in water, spores germinate directly by forming
germ tubes, or more commonly by producing many small motile
spores known as zoospores. The zoospores are responsible for

SItalic figures in parentheses refer to Literature Cited.

Florida Agricultural Experiment Station

most, if not all, of the new infections. After they are liberated
by the parent spores and lose their motility, zoospores form germ
tubes which grow into the plant through the stomata or the
lenticels. The fungus grows into the tissues of the plant and
feeds upon the cells and kills them. The life cycle is completed
when the parasite again produces its conidiophores and spores
on affected parts of the plant (51).
Spores germinate indirectly by liberating zoospores at tem-
peratures which range between 360 and 770 F., the optimum
being 540 to 550 F. Germination occurs in the field in drops
of dew or rain on the potato foliage. Under optimum conditions,
spores may germinate within 45 minutes but under ordinary
conditions they require several hours. Direct sunlight does not
retard germination unless the temperature is above optimum.
Spores die within 6 to 24 hours in dry air, and a frost that kills
potato leaves also kills the spores (51).
Spores are scattered from diseased plants to healthy ones by
wind; splashing rain, tools used in cultivating the potatoes,
inses m_ n and animals moving through the feld. Spores
whichTand on the healthy leaves and stems germinate and infect
them. When spores are washed from blighted plants into the
soil and come in contact with tubers, tuber rot develops if the
soil is wet enough. If potatoes are dug during rainy weather
before the tops have died, the freshly-dug tubers become inocu-
lated with the fungous spores from partially dead stalks and
leaves, and the disease may appear in the tubers after they
are placed in storage or are shipped to market (8).
'Potato leaves become infected through the stomata on either
their upper or lower surface, mostly the lower. In the saturated
atmosphere of a moist chamber where plants were held at tem-
peratures between 730 and 810 F., infection became visible on
the leaves within 2 to 3 days after spores were atomized on
them (51). During successive days of rainy weather when
leaves are constantly wet and the temperatures are favorable,
the disease may appear on leaves exposed to infection in the
field as soon as it does on plants held in a moist chamber. How-
ever, under ordinary conditions, 6 to 10 days are usually re-
quired for infection to occur and for the disease to develop
enough to be seen on the leaves (51). The disease has been
most destructive in Florida during wet growing seasons. At
Hastings it has caused maximum losses when the rainfall has

Potato Diseases in Florida

ranged between 5.56 and 8.31 inches during the last 30 to 40
days of the growing period (22).
Symptoms.-Leaves, petioles, flower pedicels, stems and tubers
of a potato plant may be attacked by late blight. It first appears
on the leaf as water-soaked, irregularly shaped spots which
turn light green and then purplish or dark brown as the cells
are invaded and killed by the fungus. The spots are usually
surrounded by a water-
soaked, light green ring
which includes the area
newly invaded by the
fungus as it advances in
the leaf (Fig. 3). When
plants remain wet with
dew or rain for several
hours, a grayish-white
mold consisting _spore-!
bearing structures and
masses of spores of the
fungus appear on the
lwer sides of infected
eaves and sometimes on
heir upper surfaces.
After the plants dry the
fungus withers and can-
not be seen with the un-
aided eye. During dry
weather growth of the
fungus is retarded and
the spots drqynd shrivel.
During rainmy weather
the fungus grows rapidly Fig. 3.-Late blight spots on a
the fungus grows rapidly potato leaf.
and may spread through-
out a leaf within 1 to 4 days, causing it to wilt and develop a
wet rot. Leaves killed by the disease finally drop from the
plant.
The most conspicuous symptom on leaf petioles, flower pedicels
and stems is the purplish to dark brown color of the areas in-
vaded by the fungus. These parts become brittle in the affected
areas and may be easily broken from the plant. The fungus
produces its spores less abundantly on petioles, flower pedicels
and stems than on the lower sides of leaves.

Florida Agricultural Experiment Station

Late blight spots on a tuber are brownish or purplish and
slightly sunken. They vary in number and size and may cover
most of the surface of the tuber when they grow together
(Fig. 4). The disease produces a brown stain in the flesh and
may spredathroughout the tuber (Fig. 4). The discolored flesh
of affected tubers is harder to slice than healthy flesh. Diseased
tubers of crops growing in wet soil develop a soft rot and decay,
rapidly. An affected potato held in dry storage usually rots
slowly and shrinks to a wrinkled mass.

.. *

v: .. : ::

Fig. 4.-Brown stain caused by late blight on the surface of a potato (left)
and in the flesh.

Methods of Controlling Late Blight
Blight-free Seed Stock.-Late blight is transmitted in the
tubers and cannot be controlled by seed treatment. The toler-
ance allowed for late blight in U. S. No. 1 certified seed potatoes
is 6 percent by weight when no other kinds of tuber rot are
present. Therefore, the seed should be inspected carefully when
it is cut for planting and all blighted tubers discarded.
Most shipments of seed potatoes contain some tubers affected
with late blight. Seed received from the North is usually held
3 to 4 weeks on farms before being planted. If bags containing
the seed are stored in the open where they will get wet during
rainy weather, late blight will spread from the infected to the

Potato Diseases in Florida

healthy potatoes. All freshly-infected tubers cannot be detected
and discarded when they are cut for planting and some will give
rise to primary infections in the field, and thereby increase the
chances for an epidemic of the disease. Passage of the disease
to healthy tubers can be prevented after the seed are delivered
to the farm by storing the potatoes in a building where they
will remain dry until the tubers are cut and the seed pieces
taken to the field and placed in the planter (23).
Disposal of Potato Waste.-Diseased tubers and scraps which
accumulate when seed are cut may serve as sources of late blight
infection because the fungus will multiply on the discarded ma-
terial and plants developing from it and spread to healthy potato
plants in nearby fields (7, 8). This source of the disease can be
eliminated by feeding the waste material to livestock or by
covering it with earth to a depth of 1 foot or more to prevent
any sprouts from growing out of the ground.
Soil Drainage.-Good drainage in potato fields is important
for several reasons, including protection against blight. Potato
plants growing in low places where water stands for several
hours after a rain remain wet longer than those in drier ground
and, therefore, are more likely to be attacked by the disease.
Good drainage will also help to prevent the development of late
blight tuber rot.
Digging Severly Blighted Fields.-Late blight tuber rot some
times causes severe losses when it develops in shipments of
potatoes which have been dug from severely blighted fields
during rainy weather. If the digging of blighted fields is de-
layed until the soil has dried out the disease will have sufficient
time to develop in the infected tubers so that most of them
will decay in the soil or can be seen and thrown out at the
packinghouse before they are shipped.
Blight-resistant Varieties.-Creation of varieties of potatoes
which are highly resistant or immune to late blight is one of
the principal objectives of potato breeders (55, 66). The first
blight-resistant varieties developed in the United States failed
to maintain their resistance after they had been grown for
several years. However, workers connected with the United
States Department of Agriculture reported recently that new
varieties possessing a high degree of blight resistance and yield-
ing ability equal to the best commercial ones should soon be
available for planting (66).

Florida Agricultural Experiment Station

Spraying and Dusting.-Spraying or dusting potato plants
regularly with fungicides that will kill the spores of the late
blight fungus will prevent infection. Many different sprays
and dusts have been used more or less successfully to control
blight in Florida. Degree of control obtained has varied with
the following conditions: 1. Severity of the disease through-
out the growing season. 2. Whether or not the disease was
well established in the field before fungicides were lied.
3. Frequency and regularity of application of sprays and dusted.
4. Kind of coverage of potato foliage obtained with sprayers and
dusters used. 5. Frequency and duration of rainy periods which
make fields so boggy that sprayers and dusters cannot be pulled
through them, necessitating abandonment of regular spraying
and dusting schedules.
Best control of late blight has been obtained when fungicides
have been applied with tractor-drawn power sprayers and dust-
ers (Fig. 1). The best sprayers are equipped with 6- or 8-row
drop booms and 3 to 6 nozzles per row. Any fungicide will
fail to control blight under humid conditions if complete cover-
age obtained. It is best to start spraying with at least
3 nozzles per row when the plants are small, then go to 4 and
finally to 5 or 6 to the row as the plants grow larger. The best
dusters are equipped to dust 4 to 6 rows with 2 to 3 nozzles per
row. Good control has been obtained when 125 to 175 gallons
of a spray or 25 to 30 pounds of a dust have been applied per
acre at each application. Most of the injury caused by tractor
wheels running over potato vines in the alleys when the plants
are being dusted or sprayed can be prevented by equipping the
tractor with vine lifters, or by providing wider wheel rows.
In the Hastings section it has been proved experimentally (33)
and demonstrated by growers that 4 to 6 applications of copper-
containing dusts applied at weekly intervals will control blight
when dusting is started before or immediately upon appearance
of the disease and about 25 to 30 pounds of dust per acre are
used at each application, except during those years when ex-
cessive rainfall during the latter part of the growing season
favors rapid development and spread of the disease.
In Dade County prior to the 1943-44 season copper fungicides
such as Bordeaux mixture, wettable cuprous oxides, copper
oxychloride sulfates, basic copper sulfates and copper-lime dusts
gave good control of late blight when 6 to 7 applications were
made at 6- to 7-day intervals during the growing season begin-

Potato Diseases in Florida

ning when the plants were 6 to 8 inches tall. But, during he
1943-44 season, when late blight became widespread and destruc-
tive several weeks earlier than usual, copper sprays and dusts
failed-o control it, even in those fields sprayed twice a week
for 6 to 8 weeks. Dithane,3 a new organic fungicide, when com-
bined with zinc sulfate and lime, gave outstanding control of
blight under these conditions. The combination dithane-zinc
sulfate spray was used at Homestead for the first time that
season and proved to be greatly superior to dithane alone, which
had previously been tested as a potato fungicide (62). Since
then almost all potatoes in Dade County have been sprayed
regularly with the dithane-zinc sulfate. With very few excep-
tions results were excellent. It is now used extensively- in other
potato-growing sections of the State.
The effective dosage of dithane was determined experimentally
to be 11/ pounds of the active constituent in 100 gallons of water
with zinc sulfate and lime added. Dithane was first made avail-
able for use in commercial potato fields as an amber colored
liquid containing approximately 30 percent of the active con-
stituent. Two quarts of this liquid, weighing about 10 pounds
per gallon, is equivalent to 11/2 pounds of pure dithane. The
spray is prepared with 2 quarts of the liquid dithane, 1 pound
of zinc sulfate and 1/ piou'd of hydrated lime added to each
100 gallons of water, in the order named. The dithane-zinc
sulfate spray should be applied once a week during periods when
conditions are favorable for blight infection, since the fungicide
loses much of its effectiveness after 8 to 10 days.
Zinc salt derivatives of dithane (factory made) performed as
well as dithane-zinc sulfate (tank mix) in the 1945-46 season,
and do not require the addition of zinc sulfate and lime. Other
organic fungicides have, in experimental plots and in limited
trials by growers, given results comparable with dithane-zinc
sulfate spray. Further tests may show that they are equal
to or better than dithane. Until they are proved so by tests
under severe blight conditions, growers are advised to continue
the use of dithane spray.
Tests have not proceeded far enough to establish that dithane
dust is more effective against blight than copper dusts. In
the meantime, growers who are not equipped to spray but
possess dusters are advised to continue the use of copper dusts

SDisod:um ethylene bisdithiocarbamate.

Florida Agricultural Experiment Station

and to apply them twice each week instead of once a week
during cool, rainy weather when the disease is spreading rapidly.

Early Blight
Early blight, caused by the fungus Alternaria solani (E. and
M.) J. and G., ranks as a major disease of potatoes in southern
Florida but is of minor importance in the northern part of the
State. The disease also occurs as leaf spots on tomato, eggplant
and pepper, and on black nightshade and ground-cherry (54).
In Florida, early blight appears on potatoes of the fall, winter
and spring crops as they reach maturity. Normally, it does
not attack young, actively growing foliage, but readily attacks
leaves approaching maturity and those prematurely aged by
adverse physiological or environmental factors. Such factors
as shading, insufficient fertilizer, wet soil, insect infestation,
.spray injury and cold injury often lead to the development of
early blight before the vines are mature. After the vines are
65 to 75 days old they are very susceptible to early blight.
In Florida, the fungus survives the period when potatoes are
not grown as dormant spores in old plant debris or in the soil, or
it may be carried over summer on eggplants, tomatoes and
susceptible weeds. Spores from the soil or nearby host plants
may serve as sources of primary infection on potatoes (Fig.
5 D). The spores are disseminated mainly by wind and rain.
They also are scattered by farm machinery and by insects.
A spore in contact with a leaf germinates in rainwater or dew
and enters the leaf through natural openings, or directly through
the epidermis. A small infected spot appears in 2 to 3 days.
The spot enlarges daily and a new crop of spores is produced
within 5 to 7 days from the time of inoculation when conditions
are most suitable for the development of the disease. Spore
production begins when the spots measure about 1/8 inch in
diameter and as many as 3,000 spores may be produced in a
single spot during the season. According to Rands (54), work-
( ing in Wisconsin, the disease spreads most rapidly when there
are frequent rains or heavy dews, and temperatures range
between 780 and 82 'F. However, in Florida the disease often
occurs in the absence of rain when dew is the only source of
moisture on the leaves and when the temperaturesare, much
lower than indicated by Rands. eSevere outbreaks have occurred
on the fall crop in southern Florida when the mean daily tem-
peratures during infection periods range between 620 and 70 F.

and on the spring crop when the temperatures usually range
between 68 and 740 F. during infection periods.
Tubers grown in Western and Northeastern states sometimes
become infected with early blight at digging time from spores
on dead potato vines. Bruising of tubers is an aid to infection
but is not essential. The most typical lesions have been obtained

Florida Agricultural Experiment Station

when the tubers were not bruised. Tubers become infected
and the disease develops most rapidly at temperatures ranging
from 550 to 610 F. The spots begins to appear within 10 days
after the tubers have been inoculated (34). The fungus remains
dormant in tubers held in storage at 350 F. or lower but when
removed from storage during the winter and spring and shipped
South, where they are exposed to higher temperatures, the
fungus renews its growth and the spots enlarge rapidly (31).
This is especially true of infected potatoes shipped into Florida.
Early blight has been seen on Florida-grown tubers which
were held for several weeks after digging (31). It is not
customary, however, to store potatoes over 2 to 3 days in Florida
before they are packed for shipment to market, and no com-
plaints have been received of early blight damage on Florida-
grown potatoes after they have arrived on the markets.
Symptoms.-Leaves, petioles, stems and tubers of the potato
plant may be attacked by early blight. The oldest leaves near
the bottom of the plant are the first ones affected. The disease
appears as small brown to black sunken spots usually surrounded
by a narrow yellow zone which blends with the normal green
of the leaf (Fig. 5 A). A spot resembles a rifle target, as it
consists of a series of black concentric ridges alternating with
smooth brown zones (Fig. 5 C). The spots are oval, angular
or irregularly shaped and may cover an area 1/ to 1 inch in
diameter. Some spots remain small and occupy spaces between
several small veins while others enlarge, coalesce and may cover
most of the leaf. Dead leaf tissues in the spots frequently
crack when dry and parts may fall out, giving the leaf a ragged
appearance. Leaves killed by the disease usually remain at-
tached to the plant except during wet weather. Symptoms of
the disease on the petioles and stems are similar to those on
the leaves.
Early blight spots on tubers are dark brown to black sunken
areas of different shapes and measure 1/, to 2 inches in diameter
(Fig. 5 B). Each spot may be surrounded by raised borders
of a purplish color. The number of spots on a tuber varies from
1 to many. The disease causes tubers to shrivel and become
flabby. Hard, blackened crusts of dead tissue cover the spots.
The flesh of tubers 1/ to 1/2 inch in depth beneath the spots
is black and usually is surrounded by a narrow yellow zone which
blends into the normal white of the flesh. Spots may cover
the eyes of tubers and kill the sprouts. Other fungi, such as

Potato Diseases in Florida

Fusarium spp., and bacteria may invade the spots and hasten
decay of the tubers.
Control.-Tubers severely affected with early blight should not
be used for planting. Infected seed pieces probably are a pri-
mary source of inoculum for the development of early blight
on the foliage.
Early blight has never been serious enough in the Hastings
area to justify control measures. On the other hand, in the
southern districts of the State control measures are necessary.
In view of the even greater importance of late blight the fungi-
cide program should be one that will control both diseases.
Bordeaux mixture was formerly recommended for early blight
control. In Florida, however, it does not always control the
disease. Since it is desirable to control both early and late
blights with one fungicide, it is fortunate that the dithane-zinc
sulfate spray does control both troubles. Best control of early
blight has been obtained by spraying the plants at intervals of 5
to 7 days until harvest with 100 to 150 gallons of dithane spray
per acre, depending upon the size of the plants and the row
spacing. Directions for making the spray are contained in the
section on late blight.

Southern Blight
Southern blight or sclerotium rot of potatoes is caused by
the fungus Sclerotium rolfsii Sacc. The same fungus is also
known as Corticium rolfsii (Sacc.) Curzi in its spore-forming
stage. The disease occurs throughout the Southern states and
attacks practically all vegetable crops and many different weeds
and ornamentals. It occurs scatteringly in potato fields and
seldom causes much damage. It is much more important on
peppers, tomatoes, eggplant and beans because these crops
are grown more extensively than potatoes during early fall and
late spring when soil temperatures are most favorable for
growth of the fungus.
The fungus lives in the soil on organic debris and on suscept-
ible host plants. It forms small round bodies or sclerotia which
are about the size and color of mustard seed when mature and
enable the organism to withstand unfavorable conditions. The
sclerotia germinate by forming mycelia which grow into the
stems, seed pieces and tubers with which they come in contact
(48). The fungus makes little or no growth below 460 F., does
not grow above 990 F. and grows best at temperatures ranging

Florida Agricultural Experiment Station

between 860 and 950 F. (48). The fungus is disseminated
when the sclerotia in the soil are carried by moving water or
by farm machinery. Sclerotia present in crop refuse also are
scattered when such refuse is moved from one place to another.
Symptoms.-Diseased plants can be detected by wilting of the
foliage and by the drying and brown discoloration of the stem.
White mycelium of the fungus usually may be seen on infected
parts of a stem and the sclerotia are often formed 'on the stem
and in the loose soil around it. The outer part of the stem
decays first and sloughs off with slight pressure. The plant
finally dies when the stem in the infected area is destroyed.
The disease forms cavities in seed pieces and tubers. The
fungus usually enters the seed piece through its cut surface but
may enter the tuber at any point. The disease produces a soft rot
but no offensive odor and the decaying flesh turns slightly yellow
and then brown or black when secondary organisms enter the
lesions and attack the tissues (73). Mycelial threads and
sclerotia often are present in the soil which clings to decaying
tubers, and cavities in a decaying potato may be partly filled
with the white mycelium (Fig. 6). The sclerotia usually are
present in the remains of rotted seed pieces and tubers. It

Fig. 6.-Southern blight of potatoes showing dirt, mycelium and sclerotia
of the fungus clinging to an affected tuber, left, and the same tuber sliced
to show the discoloration of the decaying flesh and white mycelium of the
fungus.

Potato Diseases in Florida

causes a rapid and complete decay of potatoes in wet soil but
develops slowly in dry soil.
Control.-No control measures have been developed for South-
ern blight.
Common Scab
Common scab, caused by Actinomyces scabies (Thax.) Gtissow,
is one of the best known potato diseases because of the char-
acteristic pustules which it produces on the skin of tubers and
its almost universal occurrence in potato-growing areas of the
world.
The scab fungus grows on the skin of the potato and produces
numerous spores which are capable of living in the soil from
one season to the next and also on tubers in storage. The dis-
ease attacks potatoes grown in soils infested with the organism
or in non-infested soils planted with scabby seed. It has also
been demonstrated that tubers of the first crop of potatoes
grown in new land may be attacked by scab even when the seed
are disinfected with formaldehyde before planting, thus indicat-
ing that the scab fungus is sometimes present in virgin soil (47).
In seasons favorable for its development scab is very severe
in Florida in infested marl soil with reactions of pH 7.5 to 8.0.
Potatoes may be severely scabbed when grown in muck and peat
soils with acid reactions and in some sandy soils testing pH 5.0
and higher (21). However, potatoes grown in sandy soils of
the Hastings section are never seriously affected with scab,
although successive crops have been planted on some land with
non-treated seed for over 40 years. The reactions of soils on
some Hastings farms range from pH 5.0 to 6.5 and are approxi-
mately the same as those of similar soils which produce very
scabby crops in other sections of the State (21). Thus, the
reaction, if above pH 5.0, is not a reliable criterion for determin-
ing the incidence of scab in Florida, but no soil with a reaction
below pH 5.0 during the spring or summer is likely to produce
a very scabby crop (21).
The disease may be 'very severe in infested soil one season
and very mild the next. Experimentally, it has been shown that
temperature, moisture and aeration of the soil, as well as pH
reaction, affect the occurrence of scab, but at present it is im-
possible to determine by soil analyses whether or not potatoes
will be scabbed when grown in soils with reactions of pH 5.0
or higher (30).

Florida Agricultural Experiment Station

Date of planting experiments on peat soils in the Everglades
indicate that temperature is a very important factor in determin-
ing whether scab will be severe or mild. With the fall crop,
the disease is more prevalent in early plantings (Table 1). The
percentage of scabby tubers is correlated with the occurrence of
high temperatures when the tubers are very young.

TABLE 1.-EFFECT OF TIME OF PLANTING POTATOES ON THE OCCURRENCE
OF SCAB IN EVERGLADES SOIL.

Similarly, observations on the spring crop in the Everglades
indicate that the late-planted potatoes suffer the heaviest losses
from scab. Potatoes planted before February 15 usually set
their tubers before soil temperatures rise to the range most
favorably for scab
development. Prob-
ably these observa-
tions explain the
relative mildness
and infreqency of
scab on potatoes
grown during the
winter months in
Dade County and in
the Hastings sec-
tion.
Symptoms.-Scab
is primarily a dis-
ease of the tubers
but may be present
on all underground
parts of the potato
Fig. 7.-A severe form of common scab. parts of the potato
plant.
The scab fungus attacks the skin of the young tuber and causes
the cells to multiply or divide in an irregular manner, thus form-
ing the scab or pustule. The disease first appears as small brown
spots on the skin. Spots may originate at any point but are

Potato Diseases in Florida

frequently found at the lenticels. They increase in size as the
tuber develops and may become more than 1/-inch in diameter.
They also vary in number and may grow together and cover
the entire surface of the tuber. Scab lesions consist of corky
tissue which is formed under the irritating action of the invad-
ing fungus. Several different types of lesions are produced, all
of which may be present on 1 tuber. They consist of a shallow
roughening of the skin, warty or swollen areas and corky pits
which may be 1/4 inch in depth (Fig. 7). In some cases wire-
worms, mites and millipeds invade the lesions and extend and
deepen them. The narcissus bulb mite is the most important
agent causing deep scab in the marl soil of Dade County.

Methods of Controlling Scab
Soil Treatment
Land which has produced very scabby potatoes should be used
for other crops, as no practical method of eliminating the scab
organism from infested soil has been developed.
Caution is advised in applying lime, ground limestone or other
alkaline materials to potato soils to adjust pH reaction, as they
tend to increase the amount and severity of scab.
Adjusting pH reactions of soils with sulfur is not recom-
mended for controlling scab in Florida.' Tests have shown that
the quantity of sulfur required to control the disease in infested
sandy soil makes the soil too acid for potato growth. Further-
more, when a soil which has been treated with sufficient sulfur
to control scab is readjusted with ground limestone to a pH
suitable for potato growth, scab returns and is as severe in the
next potato crop as it was in the crop grown on the same land
before it was treated (21).

Seed Treatment
Seed potatoes usually are treated with solutions of mercury
compounds and formaldehyde where such scab control measures
are necessary. Seed treatment experiments conducted at Home-
stead, Florida, from 1937 to 1939 with hot and cold formalde-
hyde, hot and cold mercuric chloride, acidulated mercuric
chloride, acidulated mercuric chloride-formaldehyde and seme-
san-bel showed that the hot and cold formaldehyde treatments
and the acidulated mercuric chloride treatment were almost
equally effective in controlling scab and were superior to the
other treatments. However, tests conducted at Belle Glade

Florida Agricultural Experiment Station

have shown that acidulated mercuric chloride and all other
types of mercury disinfectants promote seed piece decay in muck
and peat soils, particularly when applied to seed planted during
the early fall; therefore, only hot or cold formaldehyde are
recommended for treating potatoes which are planted in soils
of organic origin.
Seed treatment is recommended for controlling scab in Florida
with the exception of the Hastings and LaCrosse sections and
other areas where experience has shown that losses due. to
planting non-treated seed are not sufficient to warrant the ex-
pense of treating the potatoes.
Disinfection of the seed is improved if the tubers are pre-
wetted before treatment with formaldehyde and mercury com-
pounds as the water softens the scab lesions and thus increases
penetration of the chemicals. Prewetting consists of soaking
the seed (in sacks or other containers) for 2 minutes, after
which they should be allowed to drain, then piled together and
covered with a tarpaulin for 48 hours.
Hot Formaldehyde Dip.-Seed potatoes can be treated rapidly
by this method, which is used extensively in Dade, Palm Beach
and Lee counties. Generally, the seed is treated as it is unloaded
from the cars, after which it may be prepared for planting or
stored for several weeks. Efficient treating outfits will handle
up to 2,400 100-pound sacks of seed potatoes in 12 hours.
The equipment required for the hot formaldehyde treatment
consists of a large wooden or metal vat, a 10-horse power steam
boiler, and a conveyor system for moving the sacks of potatoes
through the vat (Fig. 8). Sometimes oil burners are used under
metal vats to heat the solution but these are'difficult to control
and are not recommended.
A tank 16 feet long, 3 feet wide and 3 feet deep will hold
720 gallons of solution, the conveyor belt and 8 or 9 sacks of
potatoes. The conveyor belt should be geared to move at such
a speed that a sack of potatoes is removed after being submerged
for 3 minutes. The belt should slope into and out of the tank
to facilitate handling of the sacks.
Steam under pressure can be passed through heating coils
in the bottom of the tank or directly into the solution to heat the
water to 122-124 F. An accurate dairy thermometer should be
suspended in the solution at all times and checked frequently
to see that the correct temperature is maintained. Efficiency
of the treatment is reduced if the temperature falls below 1180

Potato Diseases in Florida 23

F. and potatoes may be injured by temperatures above 1260
F. Even when heating coils are used it is desirable to have
some provision for passing steam into the solution so that the
temperature may be raised quickly if it should fall below the
correct value. Likewise, there should be a provision for adding
a cold formaldehyde solution to the vat to reduce high tempera-
tures. When the vat is in operation usually it is possible to hold
the temperature within narrow limits by regulation of a valve in
the steam line so that more or less steam can be admitted to
the coils.
The tank should be cleaned and filled 2 full with fresh solu-
tion each day. A tank of the size described will hold 720 gallons
of a solution containing 6 gallons of 40 percent formaldehyde.
When live steam is passed into the solution the concentration
of the formaldehyde becomes lower by evaporation and by dilu-
tion with the condensed steam. If heating coils are used the only
factor reducing the concentration of formaldehyde is evapor-
ation. In either case, it becomes necessary to replace the
formaldehyde lost by evaporation and dilution or by removal
in solution on wet sacks and potatoes. It has been found when
live steam is passed through the solution that the volume of the
solution remains constant, and that the addition of 11/ pints
of 40 percent formaldehyde every 15 minutes will maintain the
proper concentration. When the steam is passed through heat-

Fig. 8.-Vat, stationary boiler and other equipment used for treating
seed potatoes with hot formaldehyde.

Florida Agricultural Experiment Station

ing coils volume of the solution diminishes and it is necessary
to add both water and formaldehyde. This can be done by add-
ing 15 gallons of a solution containing 1 pints of 40 percent
formaldehyde every 15 minutes.
After the sacks of treated potatoes have been removed from
the vat they should be placed on end 1 layer deep and allowed
to drain and cool. The seed may be cut for planting at any
time after the sacks have dried.
Cold Formaldehyde Soak.-When treated by this method the
uncut tubers must be soaked 11/ hours in a solution of 1 pint
of 40 percent formaldehyde and 30 gallons of water at any
ordinary temperature. However, the soaking period can be
decreased to 30 minutes if the potatoes have been prewetted in
water. The same equipment used for the heat dip, except the
heating units, can be used to treat by the cold method. Enough
formaldehyde solution of proper strength from a reserve supply
should be poured into the tank when needed to replace that
which adheres to the potatoes and containers during treatment.
The disadvantage of the cold method is the time required.
Potatoes can be treated by the hot method in 1/10 of the time
required to treat them by the cold method. Consequently, the
cold method is seldom used to treat large quantities of seed
potatoes in Florida.
Formaldehyde is injurious to the skin and the vapors irritate
the eyes and respiratory passages. The hands can be protected
from injury by wearing rubber or oiled leather gloves. If the
treatment is made in a ventilated place, operators can work
without inhaling the vapors. Any solution left after treatment
should be poured down a drain or buried.
Acidulated Mercuric Chloride (Acid-mercury Dip).-This is
a rapid method of treatment. The stock solution is made by
dissolving 6 ounces of mercuric chloride in 1 quart of commer-
cial hydrochloric acid. This should then be added to 25 gallons
of water to make the treating solution.
A wooden barrel, tub or concrete tank should be used and the
potatoes can be dipped into the solution in sacks, wooden crates
or wire baskets painted with asphaltum paint. The solution
will corrode metal.
The potatoes should be soaked in the solution for at least 5
minutes. Soaking for as long as 40 minutes, although not neces-
sary, will not injure the seed. Twenty-five gallons of the solution
is enough to treat 40 to 50 bushels of potatoes, after which it

Potato Diseases in Florida

is likely to be too weak. It can be brought back to approxi-
mately original strength by adding 1/2 pint of the stock solution
and enough water to restore the volume to 25 gallons. After
another 50 bushels have been treated the solution should be
discarded. Unless the tubers are planted immediately they
must be spread out to dry. Treated potatoes stored while wet
may be considerably injured. Cut seed should not be treated by
this method.
The dilute treating solution is not strong enough to injure
the hands while handling the potatoes. Strong hydrochloric
acid is injurious, however, and should be handled carefully.
Mercuric chloride is a deadly poison and any of the solution left
after treatment should be buried so that animals and human
beings will not be poisoned with it. Treated potatoes should
not be used for cooking or fed to livestock.

Scab-Resistant Varieties
Progress has been made in the breeding of scab-resistant
potatoes at several places in the United States (12, 76). Thus
far, no resistant varieties have proved equal in yield and vigor
to Bliss Triumph, Sebago and other varieties commonly planted
in Florida, but it is probable that superior yielding ability and
other desirable characters will ultimately be combined with
scab resistance or immunity in varieties which can be grown
profitably in scab-infested land.

Rhizoctonia
Rhizoctonia is caused by a soil-inhabiting fungus commonly
called Rhizoctonia solani Kuihn and also known as Corticium
solani (Prill. and Delacr.) Bourd. and Galz. in its spore-form-
ing stage. It is a common disease of most vegetable crops, many
ornamentals, some tender woody plants and many weeds. Po-
tatoes, beans, cabbage, celery, lettuce, peppers, peas and tomatoes
are the principal vegetable crops attacked by the disease in
Florida (69). Losses caused by it in individual potato fields
may vary from a trace to as much as 5 percent of the crop.
The fungus lives as a parasite on growing plants or as a
saprophyte, obtaining its food from decomposing parts of plants
in the soil. Under favorable conditions it produces spores on
the leaves and stems of plants. When the spores germinate the
active stage of the fungus is produced again. It also perpetuates
itself by forming black resting bodies or sclerotia which are

Florida Agricultural Experiment Station

very resistant to cold, heat and drought and will live in the
soil or on the surface of potato tubers for many months. The
sclerotia germinate by forming mycelial threads which spread
through the soil and invade parts of susceptible plants with
which they come in contact (13). No soils in the state are
known to be free of the causal fungus. It attacks potatoes
grown in virgin soil as well as those grown in soil which has
been cropped for many years.
Development of the disease depends upon soil and climatic
conditions. It causes greatest damage to potatoes at an early
stage of growth before the sprouts have appeared above the
soil. The fungus will grow and attack the plants at soil tem-
peratures ranging from 48' to 810 F., but it grows best and
causes the greatest injury to sprouts at temperatures of 590 to
700 F. (56). Conditions which retard the growth of the sprouts
promote the development of rhizoctonia. Thus, plants growing
in cold, wet soil or in dry soil may be attacked by the disease.
Symptoms.-Rhizoctonia causes brown spots to appear on the
underground stems, roots and stolons of potato plants where
they have been invaded by the fungus (13). The organism
seldom attacks more than 5 to 10 percent of the sprouts in a
field but under especially favorable conditions may infect most
of them. Some sprouts may be destroyed completely before
reaching the surface of the soil and new ones may grow from
the seed or from the uninjured parts of sprouts which have been
partly destroyed by the disease (Fig. 9). The disease seldom
causes much reduction in stand because most seed pieces develop
sprouts which survive or escape injury and produce plants.
Usually, only a few of the roots and stolons are injured by the
disease. Most plants attacked in the early stages of growth
produce tubers, but the disease leaves scars on the affected
parts. When the plants grow old and the roots, stems and
stolons harden, the fungus is no-longer able to invade them.
If the main stem has been severely injured by rhizoctonia the
leaves may roll, particularly those in the top of the plant, and
aerial tubers are sometimes formed in some of the leaf axils.
Severely affected plants produce many small tubers, usually on
short stolons close to the stem. Some of the tubers form knobs
and are misshapen and small tubers may develop directly on
the seed piece of an affected plant.
Black sclerotia of the fungus are frequently produced on
tubers and sometimes on the roots and stolons of potato plants.

Potato Diseases in Florida

The sclerotia seen on tubers in Florida are not as large and con-
spicuous as those on Northern-grown potatoes, but it is not
difficult to find them, particularly on potatoes which have been
left in the ground a few weeks after the tops have matured. The
sclerotia-hard, dirt-like particles-stick tight to the tubers and
do not wash off. After potatoes are planted, sclerotia on the
seed pieces germinate
and form mycelial
threads which grow
into young sprouts
and roots.
The fungus may in-
vade the tubers and
cause a brown dis-
coloration and rotting
of the flesh. It also
may grow on the skin
of tubers and cause a
russetting which looks
like a mild form of
common scab. Symp-
toms of the disease on
the lower parts of a
potato plant are shown Fig. 9.-Potato sprouts affected with rhi-
in Figure 10. zoctonia. Injured areas have turned brown
Control.-F'-rmalde- and 3 new sprouts have originated from un-
ontro.- malde- damaged sections of the sprout on the right.
hyde, acidulated mer-
curic chloride and organic mercury seed treatments have not
proved beneficial in Florida, since the soils are already infested
with the causal organism (32). Tests have shown also that the
disease cannot be controlled in fields planted with seed which
are free of Rhizoctonia sclerotia.
Potato sprouts which must grow through several extra inches
of soil over deep-planted seed are more likely to be attacked
than those from shallow-planted seed. Seed should be planted
at the proper depth to insure rapid germination and quick
emergence of the young plants. Two inches is deep enough in
some sections but in the northern part of the State where plant-
ings are made during the winter and the ground occasionally
freezes, the seed should be covered with at least 3 inches of soil
to protect it from freezing.
Damage from rhizoctonia can be reduced by growing potatoes

Florida Agricultural Experiment Station

in fields which have good natural drainage or in those which are
provided with ditches to remove water rapidly during heavy
rains. If potato fields are irrigated when needed during dry
periods, the plants will maintain a normal rate of growth and
will not be injured as severely by the disease as those which
have been weakened by drought.

D

Fig. 10.-Symptoms of rhizoctonia on the lower parts of a potato plant.
A, cluster of discolored and decaying tubers near the stem; B, tuber with
knobs; C, main stem girdled and discolored by the disease; D, tubers
formed on seed-piece; E, section of rotted and discolored root; F, black
sclerotia on root and tuber; G, aerial tuber.

Sclerotinia Rot
Sclerotinia rot of potatoes is caused by the fungus Sclerotinia
sclerotiorum (Lib.) D By., which attacks many species of culti-
vated plants and weeds. The disease is important as watery
soft rot of cabbage and beans, pink rot of celery and drop of
lettuce. Sclerotinia rot has been gradually increasing in severity
in potato fields in Dade County during recent years and in the
1945-46 season caused more injury than early and late blight.
It is usually present but seldom causes much damage in other

Potato Diseases in Florida

potato-growing sections of the State. The worst outbreak of
the disease recorded at Hastings occurred in 1934 when it at-
tacked the plants early in the season and killed 15 to 70 percent
of them in several fields totaling 120 acres (17).
The fungus is dormant in Florida during the summer and at
other seasons when the weather is warm and dry. It survives
as sclerotia in old plant debris or in the soil. During cool, moist
periods the sclerotia germinate and form apothecia (Fig. 11 D)
which look like mushrooms and contain special structures known
as asci in which spores are borne. The ascospores are forcibly
discharged from the asci and, when carried by air currents and
deposited on potato leaves and stems, germinate and infect
these parts if conditions are suitable. Ascospore infections of
foliage can take place readily through dead areas in the leaf,
whether these are caused by fungi, spray burn or mechanical
means, but the ascospore germ tubes are unable to establish
infection in healthy leaf tissue under normal conditions. Once
a mycelium is formed it can readily infect all above-ground parts
of the plant through contact. The sclerotia also produce my-
celial threads which grow in or on the surface of the soil and
plant parts and initiate the disease.
Cool, rainy weather, fogs and heavy dews favor development
of sclerotinia rot. Heaviest damage has occurred to potatoes
growing in low ground which has remained wet for several days
after rains. The disease becomes most severe near the end of
the growing period when the dense foliage of potato plants pro-
vides shade which favors growth of the fungus on the lower
branches and leaves.
Symptoms.-The fungus attacks the tops of potato plants but
does not infect the tubers. The main stem may be infected at
any point, but usually is invaded at the soil line. The disease
first appears on the stem as a water-soaked lesion, followed by
growth of white mycelium of the fungus on the surface. The
epidermis in the infected area dies and turns brown as the
lesions become 2 inches or more in length (Fig. 11 A). All
except the outer, woody tissues of the stem are destroyed and
the interior becomes packed with mycelium and black sclerotia
(Fig. 11 B). The plant wilts when its water-conducting tissues
are destroyed and finally collapses and dies (Fig. 11 C). Any
of the branches and leaves of plants may become infected and
the, white mycelium usually appears on the decaying parts.
Sclerotia may be formed on the outside of invaded sections of

30

Florida Agricultural Experiment Station

leaves and branches and inside large branches. The sclerotia
are irregular in shape and measure 1/8 to 1 inch in length (10).

Fig. 11.-Sclerotinia rot of potatoes. A, Lesion on lower part of stem;
B, stem opened to show sclerotia; C, potato plant wilted by sclerotinia rot
of main stem; D, 6 apothecia produced by 1 sclerotium embedded in soil.

Potato Diseases in Florida 31

Control.-Copper and sulfur sprays and dusts applied to po-
tatoes and other vegetable crops and to the soil immediately sur-
rounding the plants have not given satisfactory control of sclero-
tinia rot in Florida (9).
Losses from the disease can be reduced by shallow cultivation
of potatoes once each week as long as this can be done without
injuring the plants. When a field is cultivated frequently,
apothecia which grow from sclerotia in the soil are destroyed
before they can produce spores which spread the disease.
In fields where potatoes and other vegetable crops have been
grown and the disease has caused severe losses, control measures
which will destroy the sclerotia in the soil are recommended.
Flooding the land with water and leaving it immersed for
a period of 4 to 5 weeks during late spring or summer has killed
the sclerotia and controlled the disease in celery fields (9).
Cyanamid treatment also has given good control (10). The
amount of cyanamid recommended for killing sclerotia is 800
to 1,000 pounds per acre for marl, muck and sandy soils.
Cyanamid should be applied uniformly with a lime distributor
or a special broadcaster and immediately disked into the soil
3 to 5 inches deep. Cyanamid is toxic to sclerotia and plant
growth when freshly-applied and it contains nitrogen in a form
which is not immediately available to plants. It should be ap-
plied far enough in advance of planting a crop for the toxicity
to disappear and for the nitrogen to be converted into nitrate
nitrogen which can be used by plants. Time required for com-
pletion of these reactions varies with the temperature, moisture
content and type of soil treated. When cyanamid is applied in
late summer or early fall after the summer cover crop has been
disked under, allow 45 to 60 days from time of treatment to
time of planting a crop (10).
Since the disease is spread by spores, it will become re-estab-
lished in treated fields within a few years from spores blown
in from neighboring, non-treated fields. Permanent control of
the disease can be secured only when all fields in a locality are
treated with cyanamid or are flooded during the summer months
(9, 10).

Fusarium Wilt and Tuber Rot
The importance of Fusarium diseases to Florida is manifest
chiefly in seed-piece decay and the discussion of that subject
should be consulted to complete the picture of these diseases.

Florida Agricultural Experiment Station

Since the conditions necessary for optimum development of seed-
piece decay are commonly encountered in Florida, it frequently
happens that seed pieces affected by Fusarium fungi decay so
rapidly and completely that germination of the seed pieces is
prevented, and symptoms of wilt in the plants are seldom seen. On
the other hand, some infected seed pieces escape complete destruc-
tion and plants with symptoms of wilt can be found in some fields.
Tuber rot, which is caused by several different species of
Fusarium, develops in the field in certain localities of the United
States and in stored potatoes (64). The disease also attacks
Florida-grown potatoes which have been kept in packinghouses
for several days after they are dug.
Wilt
Fusarium wilt in Florida is caused by 2 species of fungi,
Fusarium eumartii Carp. and Fusarium oxysporum Schlect., al-
though there are possibilities that others are concerned as evi-
denced by the fact that 61 species have been described on
potatoes (64).
Infection of potatoes by the F. eumartii wilt occurs most
readily at soil temperatures between 680 and 770 F., and is
inhibited by temperatures above 860 F. The development of
wilt caused by F. oxysporum is favored by soil temperatures
above 86 F. and by high soil moisture. Consequently, the latter
is to be expected mostly in the early fall crop, whereas the form
may be found affecting the winter crop in Florida.
Wilt caused by F. eumartii and F. oxysporum may be trans-
mitted through tubers from an infected plant. F. oxysporum
develops freely in Florida soils and infects the plants by en-
trance through the cut surface of seed pieces or through the
roots from the soil. F. eumartii is not an inhabitant of Florida
soils and the disease originates only when infected seed are
planted.
Symptoms.-Fusarium eumartii causes a wilt and stem-end
rot of potato (29). It is characterized on the foliage by a light
green mottling and irregular bronzing of the upper leaf surfaces.
The leaves slowly turn yellow, wilt and droop, hanging to the
stem by their withered petioles. Under conditions of high soil
moisture the leaves may not wilt but show yellowing, rolling and
russetting, sometimes accompanied by the formation of aerial
tubers. Internally, F. eumartii wilt is distinguished by vascular
browning throughout the plant, but particularly in the stemend
of the tubers.

Potato Diseases in Florida

Fusarium oxysporum causes vascular browning in the stem
below ground and in scattered vascular tissue throughout the
tuber. The margins of the lower leaves turn yellow, and this
symptom gradually extends to the entire plant and ends in a
complete wilt and death of the vine.

Tuber Rot
One or more species of Fusarium capable of causing tuber rot
are present in soils wherever potatoes are grown and spores of
various species are present in the air. Consequently, all tubers
soon become contaminated with Fusarium spores. Fusarium
tuber rot in the United States is caused mostly by 6 Fusarium
species, namely: F. eumartii Carp., F. sulphureum Schlecht., F.
coeruleum (Lib.) Sacc., F. trichothecioides Wr. and F. radicicola
Wr., which predominate in Western states, and F. oxysporum
Schlecht. which occurs everywhere but is very prevalent in the
southern part of the country (11). F. oxysporum sometimes
attacks seed pieces after they are planted in Florida and causes
them to rot within 2 to 3 weeks (19) and Western-grown seed
infected with F. eumartii usually rots after planting, but neither
of these 2 organisms produces a tuber rot in the field in Florida.
If sufficient moisture is present, spores of most Fusarium
species germinate and the fungi grow at temperatures ranging
between 340 and 1020 F., the optimum range for many species
being 770 to 950 F. (45). White mycelium of Fusarium species
often can be seen on the surfaces of healthy tubers when condi-
tions have been suitable for growth but infection does not occur
unless the organisms enter the tuber through lenticels, eyes,
stolons or more often through wounds or lesions caused by other
diseases. Under ideal conditions symptoms of the disease appear
in about 5 days after the tuber is infected, and if conditions con-
tinue favorable for 2 weeks or more the tuber may be destroyed.
Symptoms.-Lesions produced by most species of Fusarium
on the surface of tubers are sunken and wrinkled and partly
covered with clumps of white mycelium and spores of the fungus
(Fig. 12 A). The skin of the tuber in the infected area seldom
shows any discoloration but affected parts of the flesh are brown
to black. White mycelium and spores of the fungus usually are
interspersed with the decaying parts of the tuber. The disease
is typically a dry rot and becomes soft and mushy only when
the tuber gets wet. Secondary organisms sometimes enter the
lesions and cause bad odors and a slimy condition. Most of the

Florida Agricultural Experiment Station

interior of a potato may be rotted when the disease appears only
as a small spot on the skin. Some of the characteristic differ-
ences in the type of rot caused by F. oxysporum and F. eumartii
are shown in Figure 12 B and C.

B C.

Fig. 12.-Fusarium tuber rot of potato. A, Clumps of white mycelium
on the skin of a decaying potato; B, mycelium of F. oxysporum and dis-
coloration produced in the flesh of an affected tuber; C, decay and browning
of vascular and adjacent tissues caused by F. eumartii.

Potato Diseases in Florida

Control of Wilt and Tuber Rot
Potatoes with Fusarium rot should not be used for seed, be-
cause affected pieces may rot immediately after planting or decay
slowly and produce weak plants which wilt and die prematurely.
Information relating to care and treatment of seed for control
of seed-piece decay as given in another section of this bulletin
also applies to control of Fusarium wilt.
Since Fusaria infect tubers mainly through wounds, careful
handling of potatoes during harvesting, grading and packing
is the most effective means of controlling the rot.
Storage rot develops most rapidly at high temperatures and
can be checked by keeping the tubers in a dry, well-ventilated
place where the temperature remains constantly between 350
and 400 F.
If potatoes are shipped to Northern markets immediately
after they are dug, little Fusarium rot will develop in them,
because temperatures in the North during the winter and spring
when Florida potatoes are marketed are too low for rapid growth
of Fusaria, and most shipments are consumed before the disease
has had time to develop in the tubers.

Xylaria Tuber Rot
Xylaria tuber rot is a very minor disease of potatoes caused
by the fungus Xylaria apiculata Cooke (61). It occurs in the
marl soils of southern Florida but has not been reported on
potatoes elsewhere in tile State.
The causal fungus is common on decaying willow stumps and
other rotten wood left in the soil after the marl glades are
cleared and has been collected from rotten wood from various
other places in Florida.
Syniptoms.-The most distinctive characteristic of Xylaria
rot is the presence of slender black rhizomorphs of the fungus
which become firmly attached to the tubers. The points of
attachment become centers of infection which results in the
production of a dry rot (Fig. 13). Decayed areas are circular,
sunken, and typically light-tan colored at the centers. Secondary
invasion of the decayed areas by soft rot bacteria, soil fungi
and insects or mites frequently occurs and hastens the destruc-
tion of the tubers.
Control.-Control measures are unnecessary, since the fungus
usually disappears after the land has been farmed for several
years.

Florida Agricultural Experiment Station

A

Fig. 13.-Xylaria rot of Bliss Triumph potatoes. A, Cross-section of
tuber decayed by Xylaria; B, rhizomorphs of the fungus attached to a
tuber; C, dry rot developing at several points of attachment.
Diseases Due to Bacteria
The 4 bacterial diseases which attack potatoes in Florida are
ring rot, brown rot, blackleg and soft rot. Ring rot is a seed-
borne disease and the others are caused almost entirely by bac-
teria which are present in soils in which potatoes are grown.

Potato Diseases in Florida 37

Ring Rot

Ring rot, which is caused by Corynebacterium sepedonicum
(Spieck. and Kotth.) Skapt. and Burk., is a very serious seed-
borne disease which was first noticed in the United States in
1932 and since has spread to all of the commercial potato-grow-
ing areas of the country (4, 15).
Ring rot caused severe losses in Florida before it was brought
under control, particularly in the Hastings Section where the

damage attributed to it for the 3-year period 1937 to 1939
totalled $130,000. During the last 7 years, however, losses have
been negligible and have been due almost entirely to planting
non-certified seed affected with the disease.
The disease is perpetuated in the seed tubers and is spread
by healthy tubers coming in contact with the causal bacteria
on infected pota-
toes and on ma-
chinery and equip-
ment used in
planting, digging,
transporting and
storing the crop.
It may be spread
very extensively
at seed cutting
time when seed
pieces become
contaminated

organism. T h i s
was demonstrat-
ed at Hastings
when 42 out of
100 plants de-
veloped ring rot
when these were
Fig. 15.-Portion of a wilted branch of a potato grown from seed
plant affected with ring rot showing discolored pieces which had
and dead areas in leaves and rolling at margins of been cut from 25
leaves.
healthy tubers by
a seed cutter whose hands and knife had been contaminated by
1 smearing with decayed tissue from an infected tuber (18).
In Florida, ring rot is not carried over in the soil from one
season to the next, since it has never been known to develop
on farms where healthy seed has been planted in fields in which
the disease was present the preceding year.
Symptoms.-Ring rot may cause wilting and death of individ-
ual branches or all of the potato plant (Fig. 14). Some plants
may show no symptoms of the disease in their tops but will bear
infected tubers. The disease may attack and kill young plants,
but it is not usually noticed in the field until the last 4 to 5 weeks

Potato Diseases in Florida

of the growing period. The first symptom consists of a wilting
of the leaves and branches at their tips. Later a mottling ap-
pears in the leaves as the color fades to a pale green, then to a
pale yellow. As the leaves die they turn brown and roll upward'
at their margins (Fig. 15). Three to 4 weeks may elapse after
the first signs of wilting are noticed in the plant before the
affected parts are killed by the disease. Since ring rot appears
late in the growing season, it may be overlooked when the
symptoms are obscured by other diseases which attack and kill
the foliage of potato plants.

Fig. 16.-Tubers affected with ring rot found in seed potato bags.
Left, tuber showing cracking of the skin and outer layers of flesh; right,
a sectioned tuber showing decay and discoloration of the vascular ring
and other tissues.

Plants with 1 or more stalks or branches killed by ring rot
usually show signs of the disease in the tubers. Diseased hills
usually contain tubers varying from those apparently sound to
those completely decayed. All tubers in some affected hills
may show signs of decay but in most hills not over 1/2 of the
potatoes will be rotted. Bacteria causing the disease enter the
tubers through the stolon. Tubers only slightly affected usually
show no external symptoms, while severely affected ones may
show a cracking of the skin and a reddish-brown discoloration
at different places (Fig. 16). The disease may cause a decay
of any part or all of the vascular ring and other sections of the
tuber. Tissues affected are soft and crumbly and may be gray,

Florida Agricultural Experiment Station

cream, yellow or reddish-brown. In some tubers the outer layers
of flesh and skin decay and separate from the vascular ring,
which shows as a rotten ring when the tuber is sliced; hence
the name ring rot (Fig. 16). The center of an affected tuber
also may disintegrate, leaving only a shell (18).
Comparison of Ring Rot and Brown Rot.-Ring rot and brown
rot resemble each other closely but are caused by different or-
ganisms. Both diseases are seed-borne; but brown rot, unlike
ring rot, is not usually carried in the seed, since seed potatoes
used in most sections of the United States are produced in
localities where brown rot does not occur.
Both diseases usually appear near the end of the growing
season. Brown rot causes wilted leaves to fade to a pale green,
after which they die and turn brown. Ring rot causes mottling
in the leaves as their color fades to a pale green, then to a pale
yellow, followed by death and browning at the margins as the
leaves roll upward. Ring rot does not produce any stem symp-
toms but brown rot causes a brown discoloration of the stem
near the soil line.
Both diseases cause a ring rot of the tuber and discolor the
skin at the stolon end, the eyes and other places. Tissues
affected with ring rot are soft and crumbly, and gray, cream,
yellow or reddish brown in color. The skin and outer layers
of flesh may crack and separate from the remainder of the ring
rot tuber. Brown rot produces a brown to black discoloration
of affected tissues with no trace of red and no cracking of the
skin or separation of parts of the tuber.
When tubers and stems affected with brown rot are sliced,
white sticky masses of the causal bacteria ooze from the vascu-
lar bundles. The bacteria also ooze from the stem end and eyes
of a diseased tuber and after drying are glued to the skin. Ring
rot bacteria are not sticky and do not ooze from the tuber.
Control.-Ring rot increases very rapidly when infected seed
are planted. In 1 field at Hastings the disease increased from
a trace in the seed to 12 percent in the new crop; and in another
field from 7 percent in the seed to 45 percent in the new crop
(18). Thus, the only way to prevent losses from ring rot is to
use seed that are entirely free of it. The principal seed-produc-
ing states now maintain a zero tolerance for ring rot as a re-
quirement for certification and if certified seed are used there
is little likelihood that the disease will cause trouble.

Potato Diseases in Florida

If diseased seed are planted the crop will be affected with ring
rot and it will be impossible to pick out all diseased tubers when
the potatoes are graded and packed. Many infected tubers which
appear sound at the packing plant will decay in the packages
before arrival at the market and lower the value of the shipment.

Brown Rot
Brown rot (Bacterium solanacearum E. F. S.) or bacterial
wilt of potatoes also attacks other solanaceous plants, including
tomatoes, eggplant, peppers, tobacco and representatives of at
least 22 additional families of plants which grow in Florida.
It occurs in warm regions throughout the world and is present
in the United States in certain localities of the South Atlantic
and Gulf Coast States from New Jersey to Texas and in Penn-
sylvania, Ohio, Illinois, West Virginia and Kentucky (16, 49).
The disease is widespread in Florida and attacks susceptible
plants growing in sandy, sandy loam, clay, muck and peat types
of soil infested with the causal bacteria. It has never been
found in potatoes growing in the marl soils near Homestead.
The disease has caused considerable damage to potatoes, to-
matoes, peppers and eggplants in Florida. In some severely
infested fields it has been known to kill over 90 percent of egg-
plant and tomato plants before they produced marketable fruit.
In the Hastings section annual losses attributed to brown rot
of potatoes during the 9-year period 1932 to 1940 varied from
$25,000 to $117,000, and records show that losses caused by the
disease in some fields and parts of fields made it necessary to
abandon them for growing potatoes. It has declined in import-
ance as a potato disease at Hastings since 1940 because the
susceptible variety, Spaulding Rose, has been replaced by the
resistant varieties, Sebago and Katahdin.
The organism grows well at temperatures ranging between
770 and 97 F. Its growth is retarded at lower temperatures
and almost stops at 550 F. (49). The disease does not develop
unless minimum night temperatures are 550 F. or above and the.
maximum day temperatures are 770 F. or higher. Temperatures
are favorable for the development of the disease in some parts
of Florida during most of the year and it has been found from
March to the following January in susceptible crops growing
in infested soil. The disease has caused the greatest damage to
potatoes at Hastings when temperatures have been above nor-
mal during the latter half of the growing season. In 1935, when

Florida Agricultural Experiment Station

March temperatures were the highest recorded for 20 years,
brown rot was present in almost all potato fields in the Hastings
area and caused a loss estimated at 8 percent of the crop, ex-
ceeding that caused by all other potato diseases (16).
Sometimes the disease is very destructive in the first crop
of potatoes grown on newly-cleared land. Also it has caused
severe losses during very warm seasons on farms which have
been in cultivation for more than 40 years. Generally, how-
ever, the disease is less troublesome in old cultivated fields than
in new land. The severity of the disease varies in different
parts of a field. In some fields it occurs almost entirely on high,
dry land, while in others it is found most often in low spots
near water furrows and ditches.
The disease may be distributed locally or to more distant
places in Northern states when tomato, eggplant and pepper
seedlings and other susceptible plants grown in infested soil
are used as transplanting stock (71). Brown rot also maybe
distributed in seed potatoes which have become infected when
grown in infested soil. However, there is very little distribu-
tion of the disease in seed potatoes in Florida because.the crop
is planted almost entirely with seed produced where it does not
occur.
-\.The causal bacteria are disseminated when soil particles are
blown by wind or carried in drainage water from infested areas.
Contaminated soil particles also may be distributed to different
fields on machinery and by animals used in cultivating. It is
probable that several different leaf-eating and puncturing in-
sects are capable of distributing brown rot, since it has been
demonstrated that the disease is spread by the Colorado potato
beetle which inoculates healthy plants with the bacteria after
feeding upon diseased plants and contaminating its mouth parts
with the parasite (49).
The bacteria may enter plants through wounds made in the
roots by soil-inhabiting insects or by tools used in cultivating.
Planting corn in infested fields several weeks before potatoes
are dug may increase infection. For example, in 1 infested
field at Hastings plant infection was 29 percent higher and
tuber infection 18 percent higher in rows planted in corn (16).
Symptoms.-Brown rot first appears in the potato plant as
a slight wilting of the leaves at the ends of the branches during
the hottest period of the day. The plant recovers during the
night but the wilting becomes more pronounced each day until

Potato Diseases in Florida

finally there is no further recovery and the plant dies (Fig. 17).
Roots, stolons and parts of the stem below the soil and several
inches above it turn brown when the vascular bundles become
clogged with the bacteria which cause wilting and death by
cutting off the plant's water supply. The bacteria are white
in mass and ooze from the vascular bundles of affected parts
when they are cut or broken.
Affected tubers may or may
not show external signs of the
disease, depending upon the
stage of development of brown
rot when dug. The bacteria
ooze from the eyes and stem
end of a severely diseased
tuber and when mixed with
dirt and dried stick to the sur-
face of the tuber (Fig. 18).
The disease causes a decay and
a brown discoloration of af-
fected tissues (Fig. 18). In
advanced stages of the disease,
the brown color of affected .
parts can be seen on the sur-
face of the tuber, particularly
around the eyes and stem end.
After brown rot bacteria de- ;
stroy the vascular ring and its
surrounding tissues and break
through the skin secondary -
organisms enter and assist in Fig. 17.-Potato plant affected with
making the tuber a slimy brown rot showing wilted branches
and leaves.
mass with an offensive odor.
Plants with their tops killed by brown rot may bear healthy
as well as diseased tubers. Others showing no signs of the dis-
ease in their tops may produce diseased tubers, and plants with
both wilted and healthy stems may bear diseased tubers on the
wilted stems and sound tubers on the healthy ones. The disease
may cause a great reduction in yield if the plants are infected at
an early stage of growth and are killed before tuber formation
or before the tubers have grown to marketable size (16).
Control.-Brown rot can be avoided by growing the potato
crop on non-infested soil.

Florida Agricultural Experiment Station

Potatoes grown in soil known to be infested with brown rot
bacteria should not be used as planting stock. When crops are
planted with seed that has been grown in brown-rot-free areas
there is no possibility of contaminating the soil.
In Florida, experience has shown that it is best to delay dig-
ging an infected crop until the tops of the plants are dead.
The bacteria do not invade healthy tubers on dead plants. In /
crops dug late the disease usually has progressed enough so j
that most of the infected tubers that have not decayed com-
pletely before digging can be detected and removed at the pack-
ing plant. If potatoes are dug when the plants are still green
many infected tubers showing no external signs of the disease
may be packed with sound potatoes and thus lower the value
of the shipment.

Fig. 18.-Tubers affected with brown rot. Bacterial exudate and dirt
sticking to the eyes (left) and a section of a tuber showing decay of the
vascular ring and adjacent tissues.

Katahdin and Sebago varieties of potatoes are resistant to
brown rot. In infested soil they should be planted instead of
susceptible varieties such as Irish Cobbler, Bliss Triumph and
Chippewa (16).
If potatoes are grown on infested land where losses from
brown rot exceed 5 percent in susceptible varieties, treating
the soil with sulfur and limestone will be the most profitable
means of controlling the disease. This treatment has given
almost perfect control where used on sandy land and has proved

Potato Diseases in Florida

to be good for 5 years after it was made (20). It consists of
an application of the proper amount of sulfur to the land during
the spring or summer to make the soil very acid and thus kill
the brown rot bacteria. In the fall 6 to 8 weeks prior to plant-
ing potatoes the sulfured land is treated with sufficient standard-
mesh ground limestone to counteract the acidity and produce
a reaction favorable for plant growth. This treatment is not
recommended on marl, muck, peat or clay types of soil.
The upper 6 inches of the soil should be adjusted to pH 4.0 or
lower to kill the brown rot bacteria. An application of 50 pounds
of sulfur per acre lowers the reaction of sandy soils of the Hast-
ings, Florida, area 0.15 to 0.2 on the pH scale, and the total
quantity to use on each acre to control the disease varies with
the initial reaction of the soil, as is shown in Table 2.

TABLE 2.-APPROXIMATE QUANTITIES OF SULFUR TO BE APPLIED PER ACRE
TO SANDY SOILS OF DIFFERENT REACTIONS TO ADJUST THEM TO PH 4.0
OR LOWER FOR BROWN ROT CONTROL.

The pH reaction may vary greatly in different locations in
fields depending upon different soil types present, season of the
year and topography of the land. Consequently, samples should
be taken from the upper 6 to 8 inches of the soil in representa-
tive areas of the field immediately before treating and pH read-
ings made so that the amount of sulfur required to adjust the
reaction of all areas to pH 4.0 can be calculated. A diagram
of the field should be made before soil samples are drawn and
the location of each sample and its pH reading, together with
the necessary quantity of sulfur, noted on the diagram which
will serve as a guide when the field is treated.
Commercial flowers of sulfur containing an inert material
which prevents lumping is preferred and the percentage of this
material should be considered when calculating the quantity
needed. Sulfur containing lime or any other alkaline material
should not be used. Powdered sulfur is injurious to the eyes
and respiratory passages. Persons applying it should work on
the windward side and wear a respirator with goggles.

Florida Agricultural Experiment Station

Tests have shown that calcium and dolomitic limestone pro-
duce approximately the same changes in soil pH reactions.
Since calcium limestone is cheaper than the dolomitic form, cost
of treatment will be less if the former is used. A mixture of
approximately 3 parts of calcic limestone to 1 part dolomite
would be desirable because the dolomite would supply mag-
nesium to replace that lost through leaching following the sulfur
treatment. No attempt to economize on fertilizer should be
made the year following the sulfur application because increased
leaching resulting from the sulfur treatment has already lowered
the natural fertility to some extent. At Hastings, maximum
potato yields have been produced in sulfured soils readjusted to
pH 5.2 by treatment with 3,000 pounds of ground limestone per
acre. On the other hand, such crops as tomatoes, eggplant and
peppers do best in soils testing pH 5.5 to 6.0; consequently, more
limestone must be used to adjust soil reaction for these crops
than is needed for potatoes. Table 3 shows the approximate
quantities of limestone to be applied to light, sandy soils
of the Bladen, Scranton, and Leon types to change the re-
action from pH 4.0 to higher pH values. If a soil is still
too acid 6 to 8 weeks after it is limed, another light application
of limestone can be made and the process repeated until the
desired pH is obtained.

TABLE 3.-APPROXIMATE QUANTITIES OF LIMESTONE PER ACRE NEEDED TO
CHANGE THE SOIL REACTION FROM PH 4.0 TO HIGHER PH VALUES.

The land should be plowed and disked before it is treated with
sulfur and limestone. Both materials should be distributed uni-
formly on the land either by broadcasting or with a lime dis-
tributor and thoroughly incorporated with the soil by several
diskings.
A cover crop of cowpeas or crotalaria should be grown on sul-
fured land during the summer. These crops will do best if they
are planted immediately after the soil is sulfured.
Uramon and ammonium thiocyanate applied to soil at rates of
1,000 pounds per acre have controlled brown rot in North Caro-
lina (52). However, these materials are not recommended for

Potato Diseases in Florida

controlling the disease in Florida, inasmuch as no control studies
have been made with them on any infested soils in the State.

Blackleg
Blackleg (Erwinia carotovora (Jones) S. A. B.) is the most
common bacterial disease of the potato. The same and related
strains of the blackleg bacteria cause a soft rot of many vege-
tables, such as cabbage, carrot, celery, lettuce, pepper and to-
mato. The same organism also causes a soft rot of potatoes
but, since the symptoms of soft rot and blackleg differ, they
will be discussed separately in this bulletin.
Blackleg is most destructive in Florida when the soil remains
wet during most of the planting period. Most of the seed may
be destroyed by the disease if fields are flooded by heavy rains
within a few days after the potatoes have been planted. Under
normal conditions the disease occurs scatteringly in potato fields
and it seldom affects more than 15 percent of the plants and
usually less than 5 percent in individual fields.
Blackleg is transmitted through infected seed potatoes. After
an infected seed piece is planted and germinates the bacteria
spread into the stem of the plant and produce the disease. How-
ever, experiments conducted in Florida and other states have
shown that very little blackleg, probably less than 1 percent, is
transmitted in seed potatoes because most blackleg-infected
tubers are discarded before planting (5, 41).
Most of the blackleg in potato fields is due to infection of the
seed after it is cut (41). Blackleg bacteria are present in most
soils and infect potato seed pieces after they are planted. In-
fection is greatest in wet weather because excessive soil mois-
ture excludes oxygen from the soil and prevents formation of
a protective layer of cork cells on cut surfaces and on other
wounds of seed pieces. The blackleg bacteria enter wounds
which have not healed and initiate the disease (41, 42, 43).
Seed-corn maggots which are sometimes present in Florida
potato fields may invade seed pieces. The maggots become con-
taminated with blackleg bacteria which are present in the soil
or on the surface of seed pieces and carry them into wounds
which they make as they burrow into the seed (6).
Symptoms.-Any part of the potato plant may be affected with
blackleg. Seed pieces turn black and usually are destroyed com-
pletely by the disease. A foul smelling rot develops when secon-
dary organisms are present in decaying seed pieces.

Florida Agricultural Experiment Station

Tops of plants generally show symptoms of blackleg before
it is noticed on the stems. The disease checks the growth of
the plant and the leaves curl upward and turn yellow. The
plant wilts at first and later dies after the water-conducting
tissues in the stem are destroyed. Young plants are often
killed by the disease before any tubers are formed. Affected
plants can be distinguished from healthy ones by their rolled
and wilted leaves, yellow color and smaller size (Fig. 19).
That part of the stem nearest the seed piece decays first.

Fig. 19.-Blackleg plant of the Sebago variety with its leaves rolled,
wilted and discolored.

Potato Diseases in Florida

Decay gradually spreads up the stem to an inch or more above
the soil line and the affected part turns dark brown to black;
hence the name blackleg (Fig. 20). In wet weather when the
disease develops .rapidly, the affected part of the stem exposed'
to sunlight may be light green or greenish brown at first and
then turn black when the stem dries. Roots and stolons are

browned or blackened by the disease. An affected plant can
be pulled from the soil easily because the lower part of the
rotted stem and roots separate readily.
The disease causes a soft, mushy, brown to black rot of the
tubers (Fig. 21). Some affected tubers show no discoloration
when first removed from the soil and cut open, but the decayed
areas soon darken on exposure to the air. Affected tubers of
plants growing in wet soil decay rapidly, while those in dry soil
rot more slowly. The disease
may appear in mild form as a
slight decay and discoloration of
the tissues at the stem end of a
tuber or only as a browning of
the vascular bundles near the
stem end.
Control. Since blackleg is
transmitted through the seed,
tubers with signs of the disease
should not be planted.
Care should be taken to pre-
vent rot from developing in seed
between the time it is cut and
Slanted, because seed pieces
which show decayed spots may
be destroyed by blackleg after
they are planted. For best re-
sults, seed should not be cut
Fig. 21.-Potato cut lengthwise over 2 days and preferably not
to show decay and discoloration of oe a eoe i pat
flesh caused by blackleg. over 1 day before it is planted.
Ordinarily, cut seed can be stored
for several days without much rot developing in it, if it is kept
under shelter where it will not get wet, and if sacks, baskets
or crates containing seed are stacked so that air can circulate
between them.
Further information on the care and planting of seed to pre-
vent its destruction by blackleg bacteria and other rot-producing
organisms is given in the section on seed-piece decay.

Soft Rot
Soft rot, which is caused by Erwinia carotovora (Jones) S. A.
B., is usually the most important decay of washed early potatoes
(53), but occurs also on potatoes in all of the commercial potato-

Potato Diseases in Florida

growing areas in the country (63). Every year soft rot is more
or less troublesome in potatoes shipped into Florida for seed
purposes.
It has caused serious losses in all potato-growing sections in
Florida and was the most serious decay of washed potatoes in the
Lower East Coast section prior to the adoption of improved
methods of handling the crop in 1939 (59). Its destructiveness
has varied greatly in different seasons and in different periods
during the same season. The decay might be negligible in ship-
ments made 1 week and rise to 25 percent or more in shipments
made during the following week. Several cars shipped from
Goulds, Florida, in 1936 were beyond salvaging upon arrival
at Northern terminal markets. Since 1939 losses in transit
from soft rot have been slight in shipments from the Lower
East Coast section.
In the field soft rot occasionally may cause serious losses as
seed-piece decay and new tuber decay if the soil becomes water-
logged following excessive rainfall. It also occurs following
sunscald, fertilizer injury to seed pieces, late blight tuber rot
and other fungous infections.
The causal organism is a normal inhabitant of the soil where
its relative abundance depends largely on the supply of moisture
and of dead plant material. It can invade dead, senescent or
injured tissues only when they are wet. Fresh breaks or cracks
in potato tubers and lesions of other diseases may provide the
soft rot organism with moisture and food necessary to initiate
infection. Wet, rainy weather favors the disease either directly
or indirectly by moistening potato tissues, by disseminating
and bringing about an increase in numbers of the bacteria, by
increasing other diseases of the potato, and by altering lenticels
so that they may offer an avenue of entrance for soft rot.
Washing potatoes in the packinghouse serves to spread the
inoculum and furnishes moisture necessary for rapid growth
of the bacteria. Packaging freshly-washed wet potatoes when
the humidity is 70 percent or higher and the temperature is
moderately high (750-900 F.) furnishes conditions very favor-
able for rapid development of soft rot decay in transit or storage.
Symptoms.-The decayed tissue varies considerably in color
and consistency, depending upon temperature, moisture and light
conditions, as well as on the presence of other organisms. Af-
fected tissues are typically white to cream colored, soft and
somewhat watery if the decay develops in moist atmosphere and

Florida Agricultural Experiment Station

in dark situations. Usually a clear amber-colored liquid exudes
from the decayed area under these conditions. The line of de-
marcation between decayed and sound tissues is sharp and fre-
quently the outer portions of the tuber remain firm while the
inner are completely broken down. The decay develops a brown
color beginning at the margin and gradually extending over the
entire surface, after exposure to light and air. Decayed tissues
lose their soft consistency in a dry atmosphere and, when com-
pletely dry, may be reduced to grayish white chalky masses.
Bacterial soft rot in advanced stages of natural infection
usually is attended by foul, nauseating odors caused by secon-
dary organisms. Decay produced by pure culture inoculations
has only a slight odor.
Infections may start at cuts, cracks, bruises or lenticels (Figs.
22 and 23). It may spread by contact from decayed to healthy

tubers when potatoes are packed or piled together. In early
stages, lenticel infections appear as raised watery swellings up
to 1/4 inch in diameter around the lenticels. After exposure of
the harvested tubers to a dry atmosphere such swellings dis-
appear and the affected tissues may dry to form shallow de-
pressions at the lenticels.

Fig. 23.-Soft rot lenticel infection of a potato from water-logged soil
(left) and decay and discoloration caused by soft rot in the flesh of a tuber.

Control.-Seed potatoes affected with soft rot should be sep-
arated from healthy stock and not planted. Planting in water-
logged soils or in low places where water may stand for several
hours after heavy rains should be avoided. On low-lying land,
shallow planting on high ridges is desirable to promote aeration.
Fertilizer bands should be spaced so that chemical injury of seed
pieces will be avoided.
An adequate spraying or dusting program should be followed
throughout the season to control late blight tuber rot.
Proper handling of the crop during harvest to avoid all un-
necessary wounds, cuts, cracks, bruises and sunscald is of im-
portance. The potatoes should be harvested during dry weather
to promote rapid drying and healing of such wounds that cannot
be avoided in handling.
All potatoes infected with field rot should be sorted out. If
bacterial soft rot is abundant in the crop as it comes from the

Florida Agricultural Experiment Station

field it is advisable to store the unwashed potatoes in a well-
ventilated situation for 24 to 48 hours. Aeration causes a color
change to take place in the decayed tissues, thus permitting
the ready detection of infected tubers as they are being sorted.
If the tubers are washed they should be washed with clean
water from spray jets, rather than in soaking tanks, to avoid
contamination.
It is of primary importance to dry washed potatoes rapidly
and thoroughly before or soon after they are loaded. When
atmospheric humidity is relatively low drying by natural means
may proceed rapidly enough to prevent soft rot development.
However, periods unfavorable to rapid drying of packaged wet
potatoes occur with sufficient frequency during the packing
season in Florida to make the use of artificial means of drying
essential in larger packinghouses where continuous operation
and large output are important considerations.
Two basically different methods of hastening the drying of
washed potatoes have proved effective on a commercial scale in
Florida (59, 79). The first method is to precool the load in the
car, which removes much of the excess moisture and reduces
the temperature at the same time. Two types of precooling
equipment have been used successfully. One system uses fans
at the top bunker opening of the car to draw air upward through
the ice in the bunker and force it down through the load. The
other system does not require ice in the bunker but uses mechani-

Fig. 24.-Hot-air drier in use at Goulds, Florida.

54 "

Potato Diseases in Florida

cal refrigeration and blows cold air through the doorway of
the car to circulate the air through the load and over refrigerated
coils.
The second method employs turbulent heated- air for drying
the potatoes before they are sorted or packaged. Air heated
to 1400-150 F. is forced through specially constructed driers
(Fig. 24) in the direction opposite to the movement of the
potatoes which remain in the drier about 4 minutes. Drying
with heated air has given somewhat better control of soft rot
than precooling under Florida conditions. The chief disadvant-
age of the hot air drier is its high cost of installation (60).

Diseases Due to Fungi or Bacteria
Seed-Piece Decay
The decay of seed pieces is a source of considerable loss to
Florida potato growers, particularly with potatoes planted in
September on Everglades peat soils. The production of fall-
crop potatoes in Everglades lands has become possible only as
the causes for seed-piece decay have become known and some
measures for its control instituted. Seed-piece decay is also
of importance in other areas in seasons when heavy rainfall
happens to coincide with the planting period.
Whole potatoes seldom decay when planted under normal con-
ditions of moisture or temperature. Sometimes tubers which
have been held in storage so long as to become shriveled and
partially sprouted will decay before they can establish a new
plant.
When a potato tuber is cut into seed pieces and planted im-
mediately the cut surfaces are open wounds through which
several species of fungi and bacteria are able to enter and to
disintegrate the cellular structure of the potato tissue. Follow-
ing in the path of active pathogens, which can disorganize the
tissue, many kinds of fungi and bacteria normally leading a
saprophytic existence in the soil can complete the destruction
of the seed piece within 48 to 72 hours. The decay varies from
a dry rot to a slimy soft rot, and from white to brown in color,
depending on the conditions under which it occurs and the or-
ganism or organisms causing the disease. When bacteria are
concerned the decayed seed pieces have a putrid odor. In certain
types of dry rot there is only a moldy odor.
Fungous Decay.-The fungous pathogens which can produce

Florida Agricultural Experiment Station

decay in potato seed pieces are several species of the genus
Fusarium, particularly, Fusarium oxysporum Schlecht. and Fu-
sarium eumartii Carp. (74), and the fungi causing late blight
(Phytophthora infestans (Mont.) D By.) and Southern blight
(Sclerotium rolfsii Sacc.). The first 3 frequently are found
infecting tubers which are to be used for seed, and the seed
piece thus may be infected when planted. More commonly, how-
ever, the Fusarium species and Sclerotium rolfsii infect from
the soil through the cut surfaces of the seed pieces.

Fig. 25.-A, Mycelium of Sclerotium rolfsii growing in lesion in seed
piece; B, mycelium of Fusarium oxysporum on surface of decaying seed
piece; C, skin removed to show brown color of flesh of seed piece caused
by the late blight fungus, Phytophthora infestans.

The Fusarium rots cause brown discoloration of the infected
tissues (11) and usually show the growth of a white mycelium
on the surface and in decayed pockets in the seed piece (Fig.
25 B). Fusarium rots usually are dry but become rapidly-
developing wet rots under conditions of high temperature and
moisture. The late blight fungus also produces a brown stain
in the infected tissue just below the skin of the seed piece

56

Potato Diseases in Florida

(Fig. 25 C). This rot is dry when developing slowly but may
become soft and wet and spread rapidly under warm, humid
conditions, particularly when secondary organisms invade the
lesions. The Sclerotium rolfsii decay is soft and white, and the
white mycelial strands and sclerotia of the fungus generally are
in evidence either on the seed piece or in the soil surrounding
it (Fig. 25 A).
Bacterial Decay.-Bacterial decay of potato seed pieces may
accompany any of the fungous rots, or certain bacterial patho-
gens may initiate the infection and bring about a complete
disintegration of the affected seed piece. Bacteria of the black-
leg or soft rot group generally typified by Erwinia carotovora
(Jones) S. A. B. are the principal bacterial pathogens. Bacillus
mesentericus (Fltigge) Migula is also 1 of the bacteria capable
of infecting potato seed pieces.
Blackleg bacteria produce a rapid, soft, white decay of potato
tissue (5). Other bacteria of the same group may produce
creamy, yellowish or brown decay in the seed piece, but do not
cause blackleg. Under Florida conditions the blackleg bacteria
frequently rot the seed piece so completely that germination
does not occur and blackleg symptoms are not observed. The
color of the decayed tissue varies from white to brownish or
black as the oxygen supply is increased (42). When oxygen is
limited the decayed tissues remain white.
Control.-The fate'of an unprotected potato seed piece in the
soil is likely to be disastrous. However, when properly handled
the potato seed piece develops a very satisfactory mechanism
as a protection against pathogenic organisms and the whole
tuber is not likely to decay under reasonably normal soil condi-
tions. This is because the tuber is protected by several layers
of thick-walled suberized periderm cells through which the fungi
cannot readily penetrate. The cortical tissue of a potato tuber
when cut-has the ability to regenerate a new protective layer-
called a wound periderm. The regeneration of the protective
layer is rapid in sound tubers but is likely to be very slow or
non-existent in old, shriveled tubers. Within 24 hours after a
cut has been made in a sound potato the walls of the surface cells
are blocked by the deposition of fatty substances which retard
the loss of water. In the next 24 to 48 hours the cells below
the blocked surface begin to divide and lay down new cross walls.
As this process continues the walls become suberized and form
a protective layer as efficient as the original periderm. The

Florida Agricultural Experiment Station

wound periderm forms most effectively under warm, moist con-
ditions.
Under normal conditions the cut surfaces are sufficiently pro-
tected after 24 hours that the seed pieces may be planted without
much danger of becoming infected. However, if heavy rains
occur shortly after planting the supply of oxygen in the soil is
limited and the production of wound periderm cells in the potato
tissue is hindered. This leaves the way open for fungous and
bacterial invasion. In partially flooded soils soft rot bacteria
are especially active.
Potato tubers intended for use as seed stocks may be injured
in ways which will hinder the normal production of a wound
periderm. Sometimes when they arrive in Florida potatoes have
been subjected to high temperatures and close packing in the
cars. Such potatoes exhibit symptoms of black heart and will
rot when planted. Mercury seed treatments intended for scab
control cannot be used prior to the ethylene chlorhydrin dor-
mancy treatment. To do so encourages seed-piece decay, prob-
ably through an inhibitory effect of the mercury compounds
on wound periderm formation. The ethylene chlorhydrin treat-
ment may itself become destructive to seed pieces when the
concentration exceeds 11/3 percent, or when the temperature
exceeds 90' F. during the period of treatment. Following such
injuries, decay of planted seed pieces is particularly rapid and
usually is of the bacterial soft rot type, although if the seed
stock carried Fusarium-infected tubers, the Fusarium rots may
become destructive.
The control of seed-piece decay depends upon the adoption of
several practices. Tubers obviously diseased should not be cut
for seed. When it is necessary .to put sound tubers through
the hot-formadehyde treatment for control of scab, surface-
borne rot fungi and bacteria also are controlled. The seed pieces,
after cutting, should be placed in slatted wooden boxes or mesh
bags and loosely stacked in a well-ventilated shady packing shed
where they should remain for 24 to not over 30 hours before
being planted. Where the ethylene chlorhydrin treatment is
used it is best to wait 16 to 24 hours after cutting before the
seed pieces are dipped. In September humidity and temperature
conditions favor rapid wound periderm formation in cut seed.
The seed should not be exposed to the sun, drying winds or rain
when it is taken to the field, but should be sheltered until it is
placed in the planter. If the soil is moist, loose and well aerated

Potato Diseases in Florida

the seed pieces will do best when planted at a depth of 4 to 6
inches in peaty muck soil and 3 to 4 inches in marl, sandy and
clay soils. Protection against flooding of the field during tor-
rential rains must be provided through good ditches and pumps
so that no water is allowed to stand on the field during a storm.
When all of these conditions have been satisfied, a grower has
a reasonable assurance that seed-piece decay will not cause ex-
cessive losses.

Diseases Due to Viruses
Virus diseases of potatoes include many different mosaic
diseases, spindle tuber, leafroll, witches broom, yellow dwarf
and others. These diseases are spread by transferring the virus
present in the sap of a diseased plant to a healthy one. Insects,
especially aphids, which feed upon the tops of potato plants are
the principal carriers of the viruses. Grasshoppers, flea beetles,
leafhoppers, Colorado potato beetles, tarnished plant bugs and
others also transmit potato viruses. After a virus is introduced
into a plant it spreads to all parts and the disease is transmitted
in the tubers to the next crop.
Each virus disease can be recognized by symptoms produced
in plants or tubers or both. Some symptoms of virus diseases
are stunting of the plants, wrinkling, rolling and mottling of
the leaves and a reduction in the size and number of tubers,
which in some cases are elongated and cracked and show in-
ternal discoloration. With few exceptions, symptoms of virus
diseases do not appear in plants and tubers during the season
they become infected but are nearly always evident in the plants
grown from infected tubers.
Several mosaic diseases, spindle tuber and leafroll usually
can be found in Florida potato fields and when present in suf-
ficient amount the yield of the crop is reduced (44).

Mosaic Diseases
A mosaic disease is caused by a single virus or by a combina-
tion of viruses. Crinkle, rugose, virus X and mild mosaic are the
ones found most frequently in Florida potato fields. Ordinarily,
they cause little damage, as diseased plants are seldom numer-
ous enough to affect the yield. The old standard varieties of
potatoes, such as Bliss Triumph and Irish Cobbler, are more
susceptible to mosaic diseases than some of the new ones, such
as Sebago and Katahdin (27).

Florida Agricultural Experiment Station

Symptoms.-Potato plants affected with different mosaic dis-
eases manifest the same general type of symptoms. Leaf mot-
tling is a common symptom. It consists of small yellowish or
light-green spots scattered through the darker-green areas of
the leaf (Fig. 26). Affected plants are stunted. They die
earlier than healthy plants and their yields are reduced. Crink-
ling, distortion and shedding of
diseased leaves and brittleness
of the affected parts of the foli-
age are characteristic symptoms
of some mosaic diseases. Crink-
ling of the leaves is very pro-
nounced in plants affected with
crinkle and rugose mosaic and
less apparent in those affected
with virus X and mild mosaic.
Veins on the under sides of
leaves of a rugose mosaic plant
often turn black, and brown to
black streaks appear on leaf
petioles, stems and leaflets. A
potato plant affected with any
mosaic disease lacks vigor and
is incapable of producing much
new foliage and tubers of a sal-
able size after it has been killed
Fig. 26.-Leaf of Sebago potato to the ground by freezing dur-
plant infected with virus X, show- -ing the early stages of growth.
ing mottling which is a character-
istic of several mosaic diseases. In most cases, symptoms of
different mosaic diseases appear
only in plants originating from tubers borne by plants which
became infected with mosaic the previous season. However, if
plants originating from healthy seed become infected with a
mosaic disease early in the growing season some of the symptoms
of that disease may appear in them before they mature and die.
Control.-Losses from mosaic diseases can be avoided by plant-
ing certified seed potatoes from fields in which proper precau-
tions were taken to prevent the spread of these diseases.

Symptoms.-A potato plant affected with spindle tuber is more
upright and smaller, has fewer stems and is more branched than
a healthy plant. Its leaves may be slightly twisted and are
darker green and smaller than the normal.
Infected tubers are elongated, cylindrical or pear-shaped and
pointed at the stem end. They have a bumpy outline and their
eyes are more numerous
and shallower than those
of healthy tubers. Af-
fected tubers usually
sprout more slowly than
healthy ones. Tuber symp-
toms vary with different
varieties. Affected tubers
of Bliss Triumph are
lighter red than healthy
ones and tend to be cylin-
drical in shape, while dis-
eased tubers of Katahdin
and Sebago are generally
pear-shaped (Fig. 27).
Some tubers transmit ,
spindle tuber but show no .
symptoms of the disease.
This occurs when plants
become infected with the
virus too late in the grow-
ing season for it to affect
he size and s hap e of the Fig. 27.-Katahdin potato affected with
the size and shape of the spindle tuber.
tubers (26).
Control.-Losses from spindle tuber can be avoided by using
certified seed.
The disease is spread from infected to healthy seed by the
cutting knife, by contact of freshly-cut surfaces of seed pieces
and by the teeth of picker planters. Consequently, all potatoes
with any symptoms of spindle tuber should be thrown out and
not cut for planting.
Leafroll
Leafroll attacks all present commercial varieties of potatoes.
While more serious than spindle tuber in Florida, it is generally
of little importance because most fields contain less than 5 per-

Florida Agricultural Experiment Station

cent leafroll. Yields may be reduced as much as 63 percent
when all plants in a field are affected with leafroll but chances
are slight for much reduction in yield in fields containing only
8 percent leafroll plants (44). The disease causes a general
reduction in yield throughout an area only when most of the
acreage is planted with seed which has been produced under
conditions which have been favorable for spread of the disease
and general infection of the seed. Much of the seed used in
planting the Hastings crop in 1938 was affected with leafroll
and it caused losses estimated at 2 percent for the section and
reduced yields 50 to 60 percent in a few fields in which most
of the plants developed the disease.

Symptoms.-Seed pieces from leafroll-infected tubers sprout
slowly, remain hard during the growing season and some may

Potato Diseases in Florida

even fail to germinate and produce plants. Leafroll plants are
smaller than healthy ones. A plant originating from a seed
piece infected with the leafroll virus first shows rolling of the
lower leaves followed by rolling of those on the higher branches
until all leaves may be rolled (Fig. 28). The rolled leaves are
leathery or brittle and may show a reddish or purplish discolor-
ation. Tubers of diseased plants are attached close to the stem
and are fewer in number and smaller than those borne by
healthy plants.

sf

I f*q

Fig. 29.-Net necrosis of potatoes, showing the pattern of dead and
discolored tissues in the flesh about 1/4 inch from the stem end (left) and
near the center of a tuber.

Net necrosis is a leafroll symptom which appears in tubers of
some varieties. It consists of a network of brown to brownish-
black strands usually confined to a small portion of the flesh
near the stem end of the tuber, but if a potato is severely af-
fected the strands may extend from the stem to the bud end
(Fig. 29). It has not been observed in potatoes grown in Florida
but is sometimes present in seed potatoes obtained from North-
ern states. Net necrosis seldom appears in tubers until they
have been in storage several weeks, and it reaches its maximum

Florida Agricultural Experiment Station

development several months after the potatoes have been dug
(28). Net necrosis is not transmitted from the seed piece to
tubers of the new crop, but spindling sprouts usually originate
from that part of the tuber showing net necrosis and plants
from affected tubers usually develop leafroll.
Control.-Since leafroll is transmitted in tubers its control
consists of the use of certified seed free of the malady; Seed
should be inspected carefully at cutting time and if it contains
more than a trace of tubers with net necrosis it should be re-
placed with seed containing none.
Different varieties of potatoes vary in their susceptibility to
leafroll but none possesses any outstanding resistance. Chippewa
is very susceptible to the disease but its tubers and those of
Katahdin and Sebago are rarely affected with net necrosis (65).
Considerable progress has been made in breeding for resistance
to leafroll and new varieties highly resistant or immune to the
disease may be available for commercial production within a
few years.

Diseases and Injuries Due to Other Causes
Diseases and injuries included in this group are due to un-
known causes and to unfavorable environmental conditions or
nutritional factors.4 Some are very important and others are of
little significance. They are classified and discussed under 3
main headings. Tubers, Seed Pieces and Plants, depending upon
that part of the potato which is most seriously damaged by each
disease or injury.
Tubers
Blackheart
Blackheart is due to asphyxiation of the tissues of the tuber.
It is produced in potatoes which do not receive a normal supply
of oxygen for respiration and it develops more rapidly at tem-
peratures of 1000 F. or higher than at lower temperatures (3).
Potatoes become affected with blackheart while they are in
storage or in transit to market, as a result of overheating or
poor ventilation or both. Cut seed potatoes may develop black-
heart also during warm weather when they are stacked closely
together and left in a poorly-ventilated building for several
days.
4 Injuries caused by insects of the potato are discussed in Florida Agri-
cultural Experiment Station Bulletin 370.

Potato Diseases in Florida 65

Blackheart appears as a dark gray to black area of irregular
shape in the center of the tuber (Fig. 30). In some cases the
discoloration extends from the center to the exterior of the
tuber and a cavity may develop in the affected tissues when
they dry out. When a blackheart
potato is cut and exposed to the
air, affected tissues with a normal
color turn pink, then gray or
purplish and finally black. Af-
fected tubers may be firm but are
usually soggy and leathery, and
when invaded by bacteria develop
a wet, slimy decay.
Control.-Blackheart t u b e r s
should not be used for seed be-
cause a wound periderm is not _
formed in blackheart tissue and
affected seed pieces will be de-
stroyed by rot-producing organ-
isms. ..
Store cut seed from 24 to not
over 30 hours, as recommended
for preventing seed-piece decay.
Hollow Heart
Hollow heart consists of a sep-
Holow heat csists of a sep- Fig. 30.-Section of potato tuber
aration of the tissues in the cen- showing blackheart.
ter or heart of the tuber and it
occurs when potatoes grow too rapidly. The cavity in a hollow
heart tuber is irregularly shaped and in some cases may occupy
half of the tuber (Fig. 31). Cells lining the cavity usually
show no discoloration but may be light brown and corky in
appearance. Large tubers are more likely to be affected than
small ones. Hollow heart is not transmitted in the tubers but
is objectionable in potatoes for table stock or for seed.
Control.-Potatoes affected with hollow heart should not be
used for seed if they have large cavities in their centers because
they cannot be cut into blocky seed pieces of the proper weight
for planting.
Planting seed pieces 9 to 12 inches apart in the row, giving
competition between plants, will prevent the tubers from en-
larging too rapidly and developing hollow heart.

Florida Agricultural Experiment Station

Physiological Internal Necrosis
Physiological internal necrosis is characterized by light to
dark brown flecks and irregularly-shaped spots scattered through
the flesh of a tuber (Fig. 32). The rusty-colored areas consist

S "of dead cells which are firm,
corky and leathery in texture.
Dead and discolored tissues
are free from rot-producing
Fig. 31.-Section of potato tuber
showing hollow heart, organisms. There are no
symptoms of the disease in
the tops of potato plants or on the surface of affected tubers.
The trouble develops in potatoes which mature in dry soil during
hot weather. It has been seen occasionally in Florida in crops
which were planted in late spring or early fall. The disease
is not transmitted in the tubers, but affected potatoes should
not be used for seed as they may rot or produce weak plants.
Corky Ringspot
Corky ringspot or internal brown spot, which has been present
in European countries, the Dutch East Indies and South Africa

Potato Diseases in Florida

for many years, is of recent occurrence in Florida (1). It ap-
peared in potatoes grown on 3 farms at Hastings in 1946 and
caused losses which varied from a trace to 50 percent of the
tubers in different fields. Losses caused by corky ringspot have
been serious in some localities in foreign countries but the dis-
ease is generally considered of minor economic importance.
Cause of the disease is unknown but it has been reported as
not transmissible in the tubers (1).

S '

Other tubers which develop the disease later in the growing-

Fig. 33.-aPotto tubers affected with corky ringspot, showing ring-like
lesions on the surface (left) and discoloration of the flesh.

Symptoms.-Tubers attacked by the disease in the early stages
of growth become irregular in shape due to the development of
deep cracks and shallow corky depressions on their surfaces.
Other tubers which develop the disease later in the growing-
season are normal in size and shape. Brown, concentric rings
and arc-shaped lesions are evident on the surface of many tubers
(Fig. 33). The form of the brown discolorations in the flesh
of affected tubers varies (1). They may consist of irregular,
rusty brown blotches of different shapes and sizes, or they may
appear in the form of arcs or rings which radiate from the
center of original points of infection on the surface of the tuber

Florida Agricultural Experiment Station

(Fig. 33). The discolored flesh is corky in texture, and no fungous
or bacterial growth is evident in the dead tissues.
Corky ringspot is not the same disease as physiological in-
ternal necrosis. There are outstanding differences in the symp-
toms of the 2 diseases. Deep cracks, corky depressions, con-
centric lesions and arcs on the surface of potatoes affected with
corky ringspot are never present on tubers affected with internal
necrosis. Furthermore, discolorations of the flesh which usually
appear in the form of rings and arcs in corky ringspot tubers
are never seen in tubers affected with internal necrosis.
Control.-No control measures for corky ringspot are known.

Stem-End Browning
Stem-end browning which occurs in potatoes grown in various
parts of the United States and Canada consists of a dark brown
discoloration of the flesh in the stem end of the tuber, sometimes
appearing in streaks. It is confined to tissues close to the skin
and does not extend over 1/2 inch deep into the tuber. The cause
of the trouble is unknown. Another type of stem-end browning
has been observed in potatoes of the White Rose variety grown
in the Red River Valley of North Dakota and Minnesota. In
this type all tissues at the stem end, but seldom over an inch
in depth, are brown to brownish black, dry and hard.
Stem-end browning can be differentiated from that caused
by net necrosis, as the latter appears as a network of brown
to brownish black strands while the former is never netted.
Browning of the stem-end caused by Fusarium eumartii origi-
nates in the vascular tissues and spreads from there into the
flesh and thus differs from that caused by stem-end browning.
Stem-end browning is objectionable in potatoes for table use
but does not affect germination. Stem-end browning which is
present in potatoes grown in the Northeastern states in trans-
mitted in the seed to the next crop. However, appreciable
quantities of the disease seldom are found at digging time, and
it has never developed enough in Florida-grown potatoes to
affect their marketability.

Freezing Injury
Tubers are injured when exposed long enough to tempera-
tures of 320 F. and lower, and the nature and degree of the
injury depends upon the lowness of the temperature, length
of exposure, condition of the tubers and variety of potato.

Potato Diseases in Florida

The freezing point of potato sap ranges from 28 F. to 300 F.,
depending upon the concentration of salts, sugar and other
soluble materials in the sap (77). However, tubers will not
begin to freeze solid until they have been subjected to tempera-
tures of 260 F. or lower (39). For example, most tubers will
freeze solid or be severely injured and discolored when exposed
to 12 F. for 30 minutes, to 230 F. for 3 hours and to 260 for
12 hours (39). They will freeze more quickly when the tempera-
ture drops rapidly than when it diminishes slowly, and freezing
will begin instantly if the tubers are moved or jarred after they
have reached critical sub-freezing temperatures (39, 77).
Potatoes may be frozen in the field in Northern growing areas
if dug after the ground has been frozen several inches deep
or if kept in storage houses which have not been constructed
for protection against cold. They may be frozen also if they
are loaded for shipment or are in transit in Northern states
during very cold weather. Potatoes may even be damaged by
exposure to cold weather at Hastings and other places in the
northern part of Florida, since temperatures may drop several
degrees below the freezing point several times during the winter.
Any or all parts of a tuber frozen solid ooze water, become
soft and soon collapse when thawed in a warm, humid atmo-
sphere. When thawed in cold, dry air, the frozen parts dry and
are granular or leathery in texture. After a thawed tuber is
cut the frozen parts exposed to the air usually turn pink and
later black. When only a part of a tuber has been frozen, a
brown, corky layer may form in the non-frozen tissues ad-
jacent to the frozen part which later sloughs off, leaving the
remainder of the tuber in sound condition. In other cases fungi
or bacteria invade the frozen part after it thaws and produce
a foul-smelling, slimy rot of the entire tuber.
Tubers exposed to freezing temperatures but not long enough
for the tissues to be frozen solid show various types of internal
discoloration (39). These are evident as a dark brown to black
discoloration of the vascular ring; a network of small brown
to black streaks extending into the pith and outer tissues from
the blackened vascular ring; gray, blue or sooty black blotches
of irregular size and shape scattered throughout the interior
and apparent externally when they occur near the surface of
the tuber; and a reddish-brown discoloration occurring in ir-
regular patches in the vascular ring, cortex and pith (37).
Tubers injured extensively by exposure to low temperatures

are more shriveled and wilted than non-injured ones. Some of
the common symptoms of freezing injury found in seed potatoes
are shown in"Figure 34.
Control.-Seed potatoes which arrive in Florida during the
winter should be stored in buildings where they will not be ex-
posed to freezing temperatures.
Frozen tubers which show much blackening or discoloration
of the flesh are unfit for seed because they generally rot before
sprouting. Tubers with slight discolorations of the flesh due to
cold damage usually will sprout but are likely to produce weak
plants. Sound tubers in a shipment containing frozen potatoes
can be picked out and used safely for planting. It is usually ad-
visable to recondition such a shipment, as it may be impossible
to replace the sound tubers with undamaged seed from another
shipment in time for planting. If the damaged sacks are stored
a few days after they are delivered, the frozen tubers will have
time to soften and break down and can be detected easily and
thrown out.
Sunburn
Sunburn or greening of tubers occurs after prolonged ex-
posure to sunlight in the field or packinghouse or even after too
much exposure to electric lights while in storage. The green

Potato Diseases in Florida

color is due to chlorophyll which develops in tubers as it does
in leaves in the presence of light. Ordinarily, greening is con-
fined to tissues near the surface of the tuber but most of the
interior will turn green, yellowish-green and yellow if the po-
tatoes are exposed to light for a long time. Green parts of a
potato have a bitter taste and are poisonous to some people.
Greening mars the appearance of the potatoes but does not
injure them for seed.
Tubers of all varieties of potatoes, particularly those with
white skins, turn green very soon on exposure to sunlight in
Florida, and losses due to greening may be considerable in fields
in which heavy rains wash the soil away from the plants and
expose the tubers to light.
Control.-In Florida, potatoes are planted in ridged rows or
at ground level, depending upon the kind of culture required
to provide adequate drainage for the plants. Ridged rows should
be made as wide as possible and spaced 40 to 42 inches apart
from center to center to provide plenty of soil in which the tubers
can develop. The plants should also be kept well-ridged during
the period when they can be cultivated without injuring the
roots, as that will prevent most of the tubers which are set high
on the plants from growing out of the ground and turning green.
Potatoes should be hauled to the packinghouse the day they
are dug and picked up, since the light which enters openings in
field sacks and crates may cause the tubers to start turning
green if left in the field
more than 1 day during
clear weather.
'If it is necessary to
hold potatoes in the
packinghouse for several
days before they are
shipped to market, they
should be stored where
they will not be exposed
to sunlight or strong
electric lights.
Sunscald
Freshly-dug t u b e r s
may be scalded by ex-
posure to hot, bright
sunlight. They also scald Fig. 35.-Potato showing 1 type of dis-
coloration caused by sunscald.

Florida Agricultural Experiment Station

when overheated by remaining in contact with extremely hot
surface soil (53). Sunscald appears as blisters or as bleached
areas on the skin (Fig. 35). The flesh in a freshly-scalded spot
is soft and watery. Affected tissues are light yellow at first
and later turn brown or black. When the spots dry, they become
leathery or chalky. Symptoms of sunscald do not appear on
potatoes until several hours after they have been injured. Con-
sequently, many scalded tubers will be overlooked when they
are. packed for shipment immediately after they are dug.
Soft rot bacteria may enter scalded spots and cause rapid
decay of the potatoes while in transit if they are moist when
loaded for shipment.
Control.-Sunscald can be prevented during hot, sunshiny
days, when it is most likely to develop, by picking up the potatoes
within 10 to 15 minutes after they are dug (53). As a further
precaution, potatoes should
be hauled from the field
promptly and stored in
the packinghouse where
they will not be exposed
to direct sunlight.

Browning
Browning consists of a
light brown to a grayish
black discoloration and
shrinking of the skinned
areas on tubers (Fig. 36).
It becomes very serious on
skinned, immature tubers
which are exposed to a hot
wind in the field or while
they are being hauled to
the packinghouse. Tests
Fig. 36.-Immature tuber showing spots ha
caused by drying and browning. have sown that about as
much browning develops
on a cloudy day as on a sunny day (2). If the skinned areas
are large, the tubers lose much moisture and wither. A sticky
type of bacterial decay develops on injured parts of browned
tubers which are exposed to a moist atmosphere, or on those
which have been washed and are not dried before they are
packed and shipped to market (53). Advanced stages of brown-

Potato Diseases in Florida

ing do not become evident at the shipping point in potatoes
shipped the same day they are dug, but it develops in transit,
and when the affected potatoes reach the market they appear
very unsightly and some will be in advanced stages of decay (2).
Control.-There is no way of preventing browning of potatoes
which are very immature when dug. Digging should be delayed
until most of the tops are dead and the tubers do not skin easily
when harvested and packed for shipment.
Prompt picking, hauling and protection of the potatoes from
skinning and from long exposure to drying winds until they are
loaded for shipment to market reduce the damage from brown-
ing (53, 58).
When potatoes are dug during cool or moderately warm
weather and pass through cool weather on their way to Northern
markets, little or no browning will develop in shipments under
standard ventilation (58). When potatoes are shipped during
hot weather in late spring or summer, full-bunker initial icing
with 1 re-icing is recommended for long hauls (58).

Cracking
Cracks in potato tubers are caused by diseases, second growth,
mechanical injuries and by excessive saltiness of the soil.
Rhizoctonia, ring rot and corky ringspot cause cracking of
tubers, but the presence of other symptoms of these diseases
on affected tubers usually serve to differentiate them from
healthy tubers with
cracks due to other
causes.
Growth cracks may
appear in tubers dur-
ing seasons when the
plants are subjected to
alternating periods of
dry and wet weather.
These cracks are shal-
low and usually heal
without becoming in-
fected with rot-pro-
ducing organisms, but
they mar the appear-
ance of the potatoes.
Fig. 37.-Growth cracking and surface russet-
Saltiness of the soil ting of tuber grown in salty marl soil.

Florida Agricultural Experiment Station

following flooding by sea water after hurricanes and by the
gradual infiltration of salt water into fields adjacent to sea level
canals will cause the development of heavy surface russetting
and growth cracking if the salt content reaches a concentration
high enough to cause wilting of the growing plant (Fig. 37).
In marl soils the salt content may reach 3,000 p.p.m. in the top
3 inches by the end of the growing season without serious injury.
But if the concentration is 4,000 p.p.m. or higher, wilting occurs
and the heavy russetting and cracking becomes severe.
Cracks develop in tubers when they are handled roughly.
Tubers of Bliss Triumph crack worse at harvest time than those
of Sebago and other standard varieties (38). Cracks caused by
rough handling vary from 1/16 to 1 inch in depth and may be
an inch or more in length. They serve as points of entrance for
various fungi and bacteria which may initiate a decay of the
potato if sufficient moisture is present and the organisms come
in contact with freshly-cracked tissues which are not protected
with a wound peiiderm.
Control.-Control measures for growth cracks are the same
as for second growth and hollow heart.
Do not plant potatoes in fields which have been flooded or in-
filtrated with sea water when the concentration of salt in the
soil is higher than 3,000 p.p.m.
Cracking of the tubers as they fall from the potato digger is
more severe when they are dug while the vines are still green.
This type of cracking can be eliminated by killing the vines with
herbicides 7 to 10 days prior to harvest or by digging after the
vines have died from natural causes.
Potatoes should not be dug when fields are wet and boggy.
Tubers develop high internal pressures by absorbing moisture
from a saturated soil and are especially susceptible to cracking
(38). Careful handling will prevent most of the cracking of
potatoes when they are being dug, hauled, graded and packed
for shipment to market.

Enlarged Lenticels
Enlarged lenticels develop in tubers when potatoes are grown
in low, poorly-drained soil which has remained wet for several
days after heavy rains or overirrigation, or if the tubers are
exposed to a very moist atmosphere after digging. Lenticels
-breathing pores on the skin of the tuber-are usually small
but when enlarged they may be 1/16 to 1/4 inch in diameter.

Potato Diseases in Florida

They are white at first but become light brown and corky after
they dry and shrink and resemble common scab pustules
(Fig 38).
Tubers with enlarged lenticels usually are the first ones to
decay during wet growing seasons because large lenticels serve
as ideal points of entrance for soft rot bacteria. They also dis-
figure the tubers and make
them less salable.
Control.-Enlarged len-
ticels can be prevented by
growing potatoes in well-
drained land where water
will not collect and remain
in the low places after
rains, and by storing the
potatoes in dry places
after they are dug.
Second Growth
Second growth of tubers
develops in the field dur-
ing alternating periods of
dry and wet weather or
when potatoes are irregu-
larly irrigated. T u b e r Fig. 38.-Enlarged lenticels on a potato
growth ceases when the dug from wet soil.
soil dries out and is re-
sumed after a rain or when the field is irrigated, causing knobs
to form on the tubers at various places. Some varieties, espe-
cially Earlaine, Spaulding Rose and White Rose, are more sub-
ject to second growth than others. Knobs are sometimes formed
on tubers of plants affected with rhizoctonia. However, rhizoc-
tonia plants are seldom very numerous, while conditions re-
sponsible for the development of second growth cause many
plants throughout a field to form knobby tubers.
Control.-Timely irrigation of potato fields when needed to
supply sufficient moisture for normal plant growth will prevent
second growth of the tubers.

Dormancy
Recently harvested potato tubers will not sprout if planted
for a new crop. The period of dormancy, during which the tubers
do not respond to temperature and moisture factors favorable

Florida Agricultural Experiment Station

to sprouting, varies with varieties (57, 67). Bliss Triumph
has a dormant period of 8 to 10 weeks but the newer varieties
such as Katahdin, Sebago and Pontiac seem to have shorter
dormant periods. Dormancy is deep and prolonged in the Irish
Cobbler potato. It has been observed in experiments (46, 57, 70,
75) that the dormancy of potatoes harvested while immature
is greater than that of fully mature tubers. The period of
dormancy for any variety may be somewhat shortened by stor-
age of the seed stock under warm, moist conditions (46, 57, 78).
Chemical methods have been developed for the treatment of
potato tubers to shorten the dormant period in seed stocks
(14, 70). These treatments are .necessary where recently har-
vested seed stocks are planted for fall and early-winter crops
in the southern districts of the State. They should not be
needed in the Hastings area, nor for the spring crop in the
Everglades unless home-grown seed of the fall crop is planted.
The advantages of the treatments are that sprouting is prompt
and uniform, considerable seed-piece decay is avoided, and in
sections where early frosts are expected the crop can be matured
earlier. Higher yields and tubers of more uniform size and
quality generally are obtained from stocks treated for shorten-
ing the dormant period.
The grower has a choice of 2/treatments for breaking dor-
mancy in potato seed stocks. The ethylene chlorhydrin treat-
ment which has been used by Everglades growers is highly
effective when properly handled (70). But because it frequently
is employed in ways that are not approved it sometimes in-
creases the amount of seed-piece decay. The ammonium thio-
cyanate treatment has given results equal to those obtained with
ethylene chlorhydrin in recent experiments. Growers have ob-
tained better results with it, probably because the treatment
schedule better fits their needs and it is less subject to misuse.
When a potato seed stock is to be treated with ethylene
chlorhydrin the tubers should be cut into seed pieces the day
before the treatment is to be given. The cut seed should not
be exposed to the sun or a drying wind; nor should it be piled
too closely and allowed to heat. Good results are ordinarily
obtained if the cut seed is held for 16 to 24 hours in loosely
stacked boxes or bags in a well-ventilated shed or packinghouse.
The purpose of this pre-treatment storage is to allow the cut
surfaces to form a callus which will lessen the danger of over-
treatment and subsequent seed-piece decay. A wise precaution

Potato Diseases in Florida 77

is to have the cutters throw out all tubers showing decay.
Bacterial soft rot and Fusarium decay are spread from affected
seed pieces to clean seed pieces by contaminated cutting knives
and by contact in the boxes. The conditions of temperature
and humidity in September are especially favorable to the rapid
development of slimy decay in cut seed pieces if these precau-
tions are not followed. Seed lots that show some decay before
the ethylene chlorhydrin treatment are likely to be seriously
damaged by decay following the treatment.
Cut seed pieces held for 16 to 24 hours have formed a callus
and are ready for dipping in the ethylene chlorhydrin solution.
The solution is prepared by adding 1 gallon of anhydrous ethylene
chlorhydrin to 75 gallons of water for treating potatoes which
have been dug less than 6 weeks. The concentration may be
1 to 150 for potatoes which have been dug more than 6 weeks.
If the 40 percent ethylene chlorhydrin is used the concentra-
tions recommended for treating very dormant and partially
dormant seed pieces are 1:30 and 1:60, respectively. The cut
-tubers may be placed in boxes or bags and immersed in the
solution long enough to wet thoroughly all of the seed pieces.
After the excess solution has drained back into the container
the boxes or bags of treated seed should be loosely stacked for
8 to 16 hours. Covering is not desirable. It is best to arrange
to dip the seed pieces in the late afternoon and to hold them
over night before planting. Advantage is thus taken of lower
temperatures and there is less likelihood that the seed will be
over-treated or that rot will develop. Some growers place their
treated potatoes in an air-conditioned room where the tem-
perature can be held at 700 F. and humidity can be partially
controlled. The treated seed is ready for planting on the fol-
lowing day.
The ammonium thiocyanate treatment may be substituted
for the ethylene chlorhydrin dip. There is less danger of over-
treatment when it is used and the treatment schedule is more
adaptable to growers' practices. Response from the ammonium
thiocyanate treatment is somewhat slower than for ethylene
chlorhydrin, but final stand counts and yields generally are equal
to or higher than those with the latter treatment.
When the ammonium thiocyanate treatment is used, freshly-
cut seed pieces should be dipped in a solution containing 3
pounds of ammonium thiocyanate in 50 gallons of water. Plant-
ing may follow immediately but soinewhat better results are

Florida Agricultural Experiment Station

obtained by holding the treated seed pieces 8 to 16 hours,
preferably overnight, before they are planted. Tubers in all
stages of dormancy respond well to this treatment.

Hair Sprout
The term hair sprout or spindling sprout is applied to a condi-
tion in which weak, spindly or thread-like sprouts arise from
apparently healthy tubers.
It is usually of minor importance in Florida. However, records
show that in the 1937-38 season, when 5,000 acres in Dade
County were planted with
Dakota seed stocks in which
S2 to 65 percent of the seed
tubers were affected, the
Estimated reduction in yield
From hair sprout was ap-
proximately 59,000 bushels.
The exact cause of hair
Sprout is unknown. Tubers
produced by hair sprout
plants develop normal
sprouts and plants (36,
59) and the disease has
been experimentally shown
not to be transmissible by
grafting (36). The occur-
rence of unusually hot, dry
weather, either while the
tubers are forming in the
field or while they are in
storage, is stated to cause
t hair sprout (68).
Symptoms. Affected
tubers generally produce
Fig. 39.-Spindling sprouts, little root the spindling, weak, rapidly
growth and a small tuber produced by elongating sprouts from all
a seed piece affected with hair sprout.
or nearly all the eyes, al-
though variations in spindliness from sprouts that are thread-
like to those nearly normal in size may occur. Occasionally a
single tuber may give rise to both normal and hair sprouts. Hair
sprouts may fail to produce a plant or develop into weak plants
that produce little or no yield. Occasionally, small tubers may

Potato Diseases in Florida

form from a short stolon directly against the seed piece (Fig. 39).
Control.-After sprouting begins, tubers showing hair sprouts
should be separated and discarded for planting.

Seed Pieces

Fertilizer Injury
Fertilizer drilled in the row too close to the seed piece may
be injurious. The nature and extent of this injury was revealed
by some observations and experiments conducted on peat soil at
the Everglades Experiment Station several years ago.
Sources of potash and rates of fertilization were studied in a
fertilizer trial in 1932. The fertilizer was applied in the row
and thoroughly mixed with the soil before the potatoes were
planted. Stand counts made when emergence was completed
showed that not only the rate of fertilization but the kind of
potash salt applied had affected the results. Poor stands were
found to be associated with decayed seed pieces. The data in
Table 4 show that while the sulfate of potash was only slightly
injurious the muriate and kainit salts were highly injurious.

TABLE 4.-PERCENTAGES OF SEED PIECES WHICH PRODUCED PLANTS WITH
VARIOUS KINDS AND RATES OF POTASH FERTILIZATION.

Since kainit contains chlorides and sulfates of potassium and
magnesium, plus a large amount of sodium chloride, an experi-
ment was set up using these salts in chemically pure form in
the amounts present in kainit when used to supply 180 pounds
of K20 per acre, and in a mixture equal to kainit. The experi-
ment was conducted with peat soil in 2-gallon stone-ware jars.
Fifteen seed pieces were planted for each treatment. The
number of decayed seed pieces found with each treatment 1
month after planting is shown in Table 5.

Florida Agricultural Experiment Station

TABLE 5.-EFFECT OF POTASH FERTILIZERS AND THEIR CONSTITUENT SALTS
ON THE DECAY OF POTATO SEED PIECES.

It is apparent that the chloride salts were more injurious than
the sulfate. Sodium chloride, an important constituent of
kainit, was as damaging as kainit or a mixture of pure salts
approximating the composition of kainit.
In a similar experiment results were similar in non-sterile
soil but in sterile soil treated with kainit or sodium chloride
only a few seed pieces decayed. In the sterile soil it was noted
that seed pieces were less turgid in soil treated with large
amounts of fertilizer than with smaller amounts. Laboratory
tests showed that seed pieces planted in moist untreated soil
gained 8.5 percent in weight by the absorption of water in 5
days. In soil treated with potassium sulfate the gain was 8
percent, but with potassium chloride or sodium chloride in the
soil the seed piece gained only 6 and 4 percent, respectively.
These data indicated that chloride salts retarded the movement
of water into the seed pieces. This hindered the formation of
the wound periderm and facilitated the invasion of the seed
pieces by fungi and bacteria.
Control.-Fertilizer injury can be prevented by limiting the
amount and kind of fertilizer which will come in contact with
seed pieces. If the rate of fertilization is to be heavy, the
fertilizer should not be placed in the drill with the seed but
distributed in the row in bands l1/ to 2 inches on either side
and slightly below the seed.
Sulfate of potash is a safer source of potassium in the fertil-
izer mixture than equivalent amounts of muriate of potash or
kainit. Chlorides in fertilizers should be avoided to prevent
injury to seed pieces.

Potato Diseases in Florida

Plants
Haywire
The term "haywire" has been applied to a diseased condition
of minor importance which occurs sporadically in potato fields
planted with seed grown in the Midwestern states.
The cause of the disease is not known. In Nebraska, where
haywire is reported to occur every year, usually in small amounts,
there is no evidence of spread in the field (38).
Symptoms.-The symptoms may appear at any time during
the development of the plant. Affected plants are dwarfed with
shortened nodes and the leaflets become erect, stiff, elongate
and sometimes rolled, finally becoming yellow with more or less
purple discoloration which is more pronounced at the tips and
margins. Aerial tubers are sometimes formed in the leaf axils
and usually only a few small gnarled tubers are produced by
plants which show disease symptoms early in their develop-
ment.
Control.-Losses from haywire can be avoided by using certi-
fied seed.
Freezing Injury
Freezes which kill the tops of potato plants and reduce yields
are 1 of the worst hazards encountered in growing potatoes
in Florida, particularly in the northern part of the State. The
crop is damaged severely by freezes at' Hastings about once
every 2 years, but in Dade County and in other places in the
southern part of the State freezes occur less frequently. Freezes
which cause the most severe damage in the northern part of the
State occur between February 1 and March 15.
Potato plants will be injured or killed by freezing when the
temperature drops to 280 F. or lower. Frozen leaves and
branches wilt when the temperature goes up during the day.
They appear water soaked at first and later turn brown or black
when they die. Several days after a freeze, light green and
yellow spots appear in leaves which have been injured but not
killed by freezing. If the temperature does not drop much
below 280 F. the leaves and branches in the top and on the sides
of a large plant will be frozen and the less exposed ones in the
center and at the base will escape injury. If only a part of the
foliage has been killed several days will elapse before the extent
of the injury can be determined.

Florida Agricultural Experiment Station

If all of the foliage of a young potato plant is killed by a freeze
and the seed piece is not injured, new sprouts may grow from
the seed or new branches may be formed by the uninjured part
of the stem below the surface of the soil and produce a new top
within 2 to 3 weeks. If only a part of the top has been killed
the plant recovers more rapidly and new foliage is formed by
the uninjured stem and branches above ground. When most of
the foliage of a plant is killed after it has formed tubers of
No. 2 size or larger, it will usually produce a few new branches
and leaves but the tubers will seldom grow any larger. An old
plant may even form more stolons and set another crop of tubers
after it has been severely frozen but it usually matures and
dies before many of the tubers grow to a salable size.
Control.-There is no practical way of protecting a field of
potatoes against a killing freeze.
Earliest planting dates recommended for the 3 principal po-
tato-growing sections of northern Florida to avoid maximum
damage from freezes are: Hastings, January 1; LaCrosse and
western Florida, February 1. These recommendations apply
only to plantings made in sandy and clay types of soil. In north-
ern Florida potatoes may not escape injury from freezes when
planted in peaty muck soil prior to April 1, as crops growing
in that type of soil at Zellwood have been severely damaged by
a freeze as late as April 6.
Seed pieces should be planted deep enough to escape injury
when the ground freezes. They have never been known to freeze
at Hastings when covered with soil to a depth of 3 inches.
Seed pieces weighing about 2 ounces each are recommended
for planting, as the larger the piece the better the recovery of
young potato plants from freezing injury. This was demon-
strated by simulating a freeze which consisted in cutting off
the tops of plants 1 week after they marked the row and at a
later stage of growth when the tubers were the size of small
marbles. Results showed that plants made better recovery
and yielded more when grown from 2-ounce pieces than when
grown from smaller ones (40).
If potato fields are irrigated when needed immediately after
a killing freeze the plants will recover more rapidly.

Water Injury
Heavy rains which saturate the soil with water may injure
or kill potato plants. Overirrigation also may damage potatoes,

Potato Diseases in Florida

particularly plants growing in rows adjacent to water furrows
6r ditches which have been kept full of water for several days.
If the water table remains within 3 to 4 inches of the surface
of the soil for a few days the roots of the plants rot and die and
the tops wilt. Most leaves appear scalded, some turn yellow
and all finally turn brown when the plants die. Plants in wet
soil wilt and die more rapidly during hot weather when the
temperatures go above 850 F. than during cool weather. Plants
which have been damaged by water and remain alive until the
soil dries out never recover completely and produce poor yields.
Young plants which have not formed tubers withstand flooding
better than those in more advanced stages of growth.
Potatoes growing in a wet soil are more likely to be attacked
and destroyed by diseases such as late blight, blackleg and bac-
terial soft rot than those growing in dry soils or in those
containing a normal supply of moisture. Furthermore, soil
organisms which help 'to maintain a constant supply of food
for plants do not function in a wet soil and the plants do not
obtain sufficient nitrogen and other nutrients required for
growth.
Control.-There are no means of preventing injury from water
in low-lying soils, such as are used for growing potatoes at Hast-
ings, when the fields are flooded with 6 inches or more of water
from rains within a period of 3 to 4 days. Protection against
all but the heaviest rains can be secured by using enough field
ditches and canals to remove water rapidly from the fields. In
the Everglades, mole drains are generally used in peat and
muck soils and also pumps for removing 3 to 4 acre-inches of
water in 24 hours.
Water should not be permitted to stand in the alleys between
rows or in water furrows any longer than necessary when the
potatoes are being irrigated.

Drought Injury
Growth of potato plants is stunted and yields are reduced
when rainfall is deficient during the growing season and the
crop is not irrigated.
Seed pieces germinate poorly when planted in soil which re-
mains dry 6 to 8 weeks after planting. Some pieces dry and
shrivel and fail to sprout while others form few roots and
clusters of sprouts similar to those seen on tubers in storage
bins late in the spring. The tips of many sprouts shrivel, die

Florida Agricultural Experiment Station

and turn brown as soon as they emerge from dry soil and are
exposed to the sun (Fig. 40). Plants grow slowly or not at all
in dry soil. Leaves at the tips of the topmost branches stop
enlarging during a prolonged drought and die and turn brown
at their margins. When the supply of moisture in the soil is
restored to normal by rains or irrigation injured plants renew
growth but never recover
entirely and produce a nor-
mal yield.
Control. Irrigation of
potato fields when needed
to supply sufficient mois-
ture for germination of the
seed and growth of the
plants will prevent drought
s1 injury and increase yields.

Hail Injury
Damage to potato plants
Sa caused by hail depends upon
the amount of hail that
falls, size of the hail stones
and stage of growth of the
plants when injured. Hail
may knock off most of the
leaves and those remain-
ing on the plant may be
full of holes. The stems
may be severely bruised
and broken also. If plants
Fig. 40.-A seed piece after 60 days are not too severely injured
in a dry soil, showing few roots and a by hail in early stages of
cluster of sprouts.
growth they will recover
and form new tops and produce marketable potatoes. But if
most of the foliage is destroyed after the plants have formed
tubers of U. S. No. 2 size or larger, little tuber growth takes
place thereafter and yields are greatly reduced.
Lightning Injury
Potato plants are sometimes injured or killed by lightning.
The area in which the plants are affected is usually circular and
under 80 feet in diameter. Plant injury is worst in the center
of a spot and decreases as the distance from the center increases.

Potato Diseases in Florida

Severely damaged plants are prostrate. The stems are shriveled
and flattened but the leaves may remain healthy and turgid
several days after the plants have been injured (72). When an
injured stem is invaded and rotted by soft rot bacteria, the
leaves wilt, the stem -turns brown or black and the plant dies.
The stem of a lightning-struck plant may be hollow a short
distance just below the ground line but most stems below the
soil surface appear normal. The roots and tubers of affected
plants show no signs of direct injury (38).

Malnutrition
Potato plants growing in non-fertilized soil or in soil which has
not received a sufficient amount of fertilizer usually show definite
deficiency symptoms (Fig. 41). They seldom produce market-
able tubers and
often die prema-
turely.
Plants showing
marked symptoms
of malnutrition are
often found at
the ends of rows
where no fertilizer
was placed in the
soil with the seed.
They also occur at
other places in the
field where the soil
was not fertilized
properly.
Control.-Malnu-
trition can be pre-
vented by applying
the right amount Fig. 41.-Potato plant affected with malnutri-
tion, showing spindling type of growth, blooming
of commercial fer- ,and rolled leaves.
tilizer of the proper
formula for growing potatoes in different types of soil as recom-
mended in Florida Agricultural Experiment Station Bulletins
295 and 353 (24, 25).
The fertilizer distributor should be in good mechanical con-
dition and operated properly. In turning around at the ends
of the field the lever controlling the flow of fertilizer from the

Florida Agricultural Experiment Station

distributor should be lowered at the proper time to place fertil-
izer in the soil with the seed at the beginning of each row.

Certified Seed
Certified seed potatoes are used in planting 95 percent or
more of the commercial potato crop in Florida. Most of the
seed used in the Hastings section comes from Maine and Prince
Edward Island and New Brunswick, Canada, while most of the
Bliss seed planted in Dade County is grown in North Dakota.
More than a dozen other states also furnish a part of the seed
to plant the Florida crop.
Certified seed produced under the supervision of reliable certi-
fication agencies are inspected 2 or more times during the grow-
ing season and at least once after the crop is dug by men trained
to recognize potato diseases. The quality of certified seed and
its freedom from disease depends upon the efficiency and honesty
of the inspectors and seed producers. Good certified seed are
true as to variety, entirely free of some of the worst seed-borne
diseases and contain no more than the tolerances allowed for
other diseases.
Some states certify only 1 market grade of seed potatoes while
others certify 2 or more grades. Practically all seed used in
Florida are of the U. S. No. 1 grade. Each seed-producing state
has its own certification tags on which a brief description of
the seed is printed. These tags are fastened to packages after
they are filled with certified potatoes. A blue tag is commonly
used to identify seed of the U. S. No. 1 grade and tags of other
colors are used for seed of lower market grades.
It is a good practice for the buyer to examine several bags
of each shipment of seed before the potatoes are unloaded from
the railroad car or removed from the warehouse where they
have been stored after being unloaded from a boat. If there
is any doubt about the grade or condition the potatoes should
be inspected by a Federal-State inspector. If the inspector's
report shows that the potatoes do not meet the grade specified
in the sales agreement, the buyer will not have to accept the
shipment.
As a further precaution, the grower should examine the seed
when it is delivered at the farm. If it is found to be seriously
affected with 1 or more diseases or is frozen it should be re-
jected and a healthy lot obtained for planting. Different lots

Potato Diseases in Florida

of seed should be cut separately and the location in which each
is planted in the field recorded. The field should be inspected
for diseases during the growing season. If plants of any lot
are found affected with ring rot, or if any lot is seriously affected
with leafroll or some other virus disease, the agency which certi-
fied the potatoes should be notified so that steps can be taken
to stop the propagation of the diseased stock for seed.

Conclusions
Diseases and injuries of potatoes discussed in this bulletin
occur in Florida and do not include all which affect potatoes
throughout the world. It is necessary to be constantly on the
lookout to prevent the introduction and spread of a new disease
in imported seed potatoes. Ring rot is an example of a very
serious seed-borne disease which was introduced into this coun-
try over 15 years ago and caused great losses before it was
recognized and means for controlling it were adopted.
Any fungous or bacterial disease of the potato can cause
serious damage when it occurs in epidemic form, but late blight,
common scab, ring rot, soft rot and brown rot are the ones
most likely to cause the greatest losses in Florida. Leafroll
is the most important virus disease and the most destructive
injuries are due to freezes, heavy rains and droughts.
Control measures recommended for the most important dis-
eases and injuries of the potato are summarized in Table 6.
Those listed for some of the diseases and injuries such as ring
rot, brown rot, virus diseases, sunscald and browning will give
almost perfect control, while others will prevent excessive losses.
Tests are constantly being made in Florida and other potato-
growing areas to find more effective and more economical means
of controlling major potato diseases. Thus, fungicides now
recommended for controlling late blight and other diseases may
be replaced by better ones within a few years. Results of re-
search work at different potato breeding and testing stations
in the United States (55, 65, 66, 76) also show that new varieties
of potatoes may soon be developed which will be immune or
highly resistant to some of the worst diseases, including late
blight, common scab and leafroll.

88 Florida Agricultural Experiment Station

TABLE 6.-SUMMARY OF CONTROL MEASURES RECOMMENDED FOR THE MOST
IMPORTANT POTATO DISEASES AND INJURIES.

Disease or Injury Control Measures
*-

Late blight

Early blight

Common scab

Use blight-free seed when obtainable.
Keep seed under shelter and dry until planted.
Destroy affected seed tubers and potato scraps or
feed them to livestock.
Provide good drainage in potato fields.
Spray or dust crops regularly with copper fungicides
or spray regularly with dithane-zinc sulfate spray.
Dig severely blighted fields during dry weather.

Do not plant potatoes in soil which has produced very
scabby crops.
Treat seed with acidulated mercuric chloride solution
or hot formaldehyde solution except as follows:
1. When potatoes are to be planted in muck or peat
soil use the hot formaldehyde treatment a.d
not the acid mercury dip treatment.
2. Do not treat seed when potatoes are to be grown
at Hastings and in other parts of the State
where scab has caused little trouble.

Plant seed at proper depth in moist soil for rapid
Rhizoctonia germination and quick emergence of sprouts.
Provide good drainage and irrigate fields when needed
to promote growth of plants.

Sclerotinia rot

Fusarium wilt
and tuber rot

Ring rot

Cultivate crop frequently during early stages of
growth to destroy spore-producing stage of causal
fungus.
Treat soil with 800 to 1,000 pounds of cyanamid 6 to 8
weeks before planting potatoes or flood land with
water and leave it immersed 4 to 5 weeks during
late spring or summer.

Discard diseased seed.
Handle tubers carefully to prevent unnecessary wound-
ing.
If potatoes are not shipped to market immediately
after they are dug, place them in cold storage
where temperatures are held at 35 to 400, F.

Use seed stocks free of ring rot.

Potato Diseases in Florida

TABLE 6.-SUMMARY OF CONTROL MEASURES RECOMMENDED FOR THE MOST
IMPORTANT POTATO DISEASES AND INJURIES--(Continued).

Disease or Injury

Control Measures

Use seed grown in brown-rot-free areas.
Plant Sebago, Katahdin or other resistant varieties
in mildly infested soil.
Brown rot Do not plant potatoes in severely infested soil unless
it has been treated with sulfur and limestone dur-
ing the summer and early fall.
Dig infected crops after the vines have died.

Blackleg

Soft rot

Seed-piece decay

Virus diseases
Mosaic diseases
Spindle tuber
Leafroll

Blackheart

Discard blackleg-infected tubers.
Plant healthy seed within 1 to 2 days after it is cut
but not in wet soil.

Discard seed tubers affected with soft rot.
Do not plant seed in wet soil.
Space fertilizer in bands to avoid chemical injury
of seed pieces.
Spray or dust crop to control late blight.
Dig crop during dry weather and handle tubers so
as to prevent unnecessary wounding and sunscald.
Store unwashed potatoes from severely affected crops
in weli-ventilated packinghouses 1 to 2 days before
grading and sorting.
Wash potatoes with clean water from spray jets.
Dry washed potatoes with heated turbulent air or
remove excess moisture by precooling them in the
car before they are shipped.

Use healthy seed potatoes.
Store cut seed in slatted wooden boxes or mesh bags
stacked loosely together in a well-ventilated build-
ing where it is protected from sun, drying winds
and rain until planted.
Do not plant seed in wet soil; provide good drainage
and install pumps when needed to remove water
rapidly from fields during rainy weather.

Plant certified seed potatoes obtained from areas
where proper precautions are taken to prevent seed
from becoming affected with virus diseases.
Discard seed tubers with symptoms of spindle tuber.
Do not plant seed affected with net necrosis.

TABLE 6.-SUMMARY OF CONTROL MEASURES RECOMMENDED FOR THE MOST
IMPORTANT POTATO DISEASES AND INJURIES--(Continued).

Disease or Injury

Freezing injury
of tubers

Control Measures

Protect seed potatoes from freezing during the winter
in North Florida by storing them in buildings until
they are planted.
Discard all frozen and discolored tubers for planting.

Earliest planting dates recommended for sections of
northern Florida to avoid maximum injury from
Freezing injury freezes are: Hastings, Jan. 1; LaCrosse and west-
of plants ern Florida, Feb. 1; and in all areas in northern
part of the State in which plantings are made in
peaty muck soil, April 1.
Plant seed pieces weighing about 2 ounces each and
cover them with at least 3 inches of soil.

Cultivate crop as long as possible to keep plants well
Sunburn ridged and tubers -covered with soil.
Sunscald Pick-up potatoes 10 to 15 minutes after they are dug,
haul them from field promptly and store them in
packinghouse where they will not be exposed to
direct sunlight or strong electric lights.

Do not dig crop when tubers are immature and skin
easily.
Handle potatoes carefully to avoid unnecessary skin-
ning.
Browning Pick-up and haul tubers from field promptly after
digging to avoid long exposure to drying winds.
During hot weather of late spring and summer, ice
cars at loading station and re-ice on long hauls to
keep the potatoes cool.

Do not plant potatoes in fields which have been flooded
or infiltrated with sea water when the concentra-
tion of salt in the soil is higher than 3,000 p.p.m.
Irrigate fields when needed.
Cracking IVarieties such as the Bliss Triumph which crack easily
as they fall from the digger should be dug after the
vines have died from natural causes or have been
killed with herbicides 7 to 10 days prior to harvest.
Do not dig crop when soil is wet and handle potatoes
carefully during digging, hauling and packing.

TABLE 6.-SUMMARY OF CONTROL MEASURES RECOMMENDED FOR THE MOST
IMPORTANT POTATO DISEASES AND INJURIES-- (Concluded).

Disease or Injury Control Measures

Use ammonium thiocyanate treatment on freshly-cut
Dormancy Iseed or the ethylene chlorhydrin treatment on seed
which were cut 16 to 24 hours previously. Hold
treated seed 8 to 16 hours before planting.

Place fertilizer in bands 1% to 2 inches on either side
Fertilizer injury and slightly below the seed.
of seed pieces Do not use fertilizers containing kainit, muriate of
potash and chloride salts.

Apply right amount of fertilizer required for grow-
Malnutrition ing potatoes in different soils as recommended in
Florida Agricultural Experiment Station Bulletins
295 and 352.

ACKNOWLEDGMENTS

The authors are indebted to E. Q. Proctor, Laboratory Assistant, Potato
Investigations Laboratory, for making, the photographs used in 28 of the
figures in this bulletin and to E. L. Felix, Assistant Plant Pathologist of
the Everglades Experiment Station, for making the photographs used in
Figure 8.
The writers also acknowledge their indebtedness to many individual
potato growers and firms for access to their fields and packing plants for
making inspections for diseases. In particular, the authors wish to thank
the following who furnished in part the facilities for research on control
of potato diseases: Hastings Potato Growers Association, G. V. Leonard,
Bruce Gray and Ralph W. Atkinson, Hastings; L. C. Conova and A. B.
Geiger, Green Cove Springs; Dale Foster, Arcadia; D. R. Cox, Gainesville;
Mrs. H. H. Wedgworth and E. H. Borchardt, Belle Glade; the Tropical
Agriculture Cooperative Association and Goulds Growers, Inc., Goulds;
and the Board of County Commissioners of Dade County.